I think there's a pile. None of this is to say that anyone "owes" someone a relationship or that an individual man isn't making some sort of a choice when he goes off the rails and stalks me for several years. (this is also about cishet men)

I also want to say that this is going to appear in individual men different amounts. If you're a man and you reflexively think "well, I don't X", that's fine. But take it on board and think seriously (possibly with a therapist) whether parts of this do exist in you the next time you're rejected or whatever. This is a rough broad cultural commentary, which is fuzzy as hell. It is also done to boys and men (by other boys and men, and sometimes by women, if that takes some of the sting away).

Most bad: Under patriarchal norms, there is a sense of ownership of a woman (a specific woman, rather than women generally) in our language of relationship. This has been more explicit in the past, but it's still there in our media, "thought leaders" etc. Obviously, no guy says it and often they can't even sus out their feeling. It happens early in courting, well before any sort of exclusivity or even much social contact. The sting of rejection sucks, sure, but that you'd take it beyond your immediate feelings and wander around after a person requires some sense of... "This person is mine". This thought pattern is pervasive and unaddressed and causes, I think, most of the violence against women (you wouldn't want your property running away from you, your toaster doesn't talk back to you etc). I think this is worth delving into more specifically than just "male entitlement to women's bodies" (which other people have written about a bunch)

Also bad: Men in our society are in a competitive hierarchy. You want to be taken seriously by your friends? Well, maybe they'd stop making fun of you if you had a hot partner. Again, this is something a lot of guys will not say, but a lot of "choosing" a partner is about what they think their friends (and powerful men around them) would say. They might be fine with someone chubby or trans or whatever, but a voice in the back of their head says "But I know Robert will make fun of me for this". This applies more the further down the hierarchy of masculinity you are, the further you are from being taken seriously in that sphere. While the first point applies to men pretty well across the board, this definitely has a classist ableist character. See the dating foibles of masc computer science students.

Bad, related to the first two: Men, as a class, feel entitled to women. Myriad reasons, as caregivers and sources of offspring. In our modern liberal nation-states, there's always an obsession with birthrates. In prior generations, familial lines and property inheritance promoted... Well, men owning women, both as a class and as individuals. However, any individual man being under the effects of this doesn't necessarily feel that entitlement. Some definitely do, see the prevalence of cat-calling. Some do without cat-calling (say, a handsy boss). Other people have written more about this and better than I could.

More innocent on the part of men, but still bad societally: Men are more isolated than women. I realise that men have more access to power and privilege than women in this society, however men are also divorced from both their internal feelings and their communities in a way women (often) aren't. If you think about the typical men's friend circle, it's often mediated almost entirely around common hobbies and a once-common space (school, uni), and the conversations tend to be surface level and avoid anything particularly rough. There's an emphasis on never backing down from a position, lest one lose their position in the aforementioned hierarchy, which means that any interpersonal emotional discussions are very high labour and high risk. This is isolating. The most acceptable way to access that sort of connection is through a romantic relationship, for men at least. Women do sometimes experience this, but to a lesser degree. Men are also raised to have few emotions. Realistically, this means they still have those emotions but do not understand them well and outwardly try to express rage, glee, or lust (the real man emotions that aren't really emotions and you can express them without anyone lest yourself doubt your manly stoicism).

Unmet social needs: I had someone get on my case about calling them social needs, but that's the psych language. It's not going to kill you like not breathing, but it will kill you like smoking and drinking. People have romantic, platonic, and physical affection needs that a lot of single men are not getting met at all. While it's not on any one woman or women generally to fix this, it is something that does need addressing. Traditionally, everyone would be a part of a community which would meet quite a lot of these needs, but we live in a very isolated society. Whatever else, these unmet needs causes a lot of extremely weird behaviour even outside of harrassing women. That isn't to say that a solid community wouldn't have any of these problems (e.g. peasant communities in many places are very patriarchal), just to say it would have less of our current weirdos.

Ancillary: Sub-section for relatively surface level things that nonetheless cause a lot of these interactions and reflect a lot of the above. However, they can be changed by producing different media etc.

• Terrible dating advice. idk, there's lots out there and where else does one turn to? Women? They're illogical and don't know what they want! (also they want different things, and this is confusing and enraging)
• Social scripts. If you're poorly socialised, you often rely on movies and "success stories" to provide you with scripts of what to do. I see this amongst isolated autistic boys a lot. They do have agency, but frankly even "woke" romance stories are not particularly good at showing how to deal with rejection.

And obviously, some men are sociopaths and do stuff because they can get away with it. But that's actually pretty rare (1 in 25? idk).

I don't really have a good answer. I know that modern "trad values" masculinity basically wants more of this (with a side dressing of idealised yeomanry). Suburbia destroys community by making us competitive (lawn measuring) and making everything very far away. We only really encounter bastard forms of community in modern society that have had to be recreated from the ground up with people who've been raised with very hierarchical military/prison rules (corporations and schools), and a lot of the stronger "communities" we encounter in urban life are reactionary. :( Be the best man you can be though.

I also don't think ceding this entire ground to reactionaries is the right choice, even if I think that most of "masculinity" is at the very least silly and arbitrary. Kinda like how I'm definitely an internationalist that would get rejected from any nationalist project, people are attached to their nations and its within "The Left"s best interest to be able to work through and address the topic without alienating a huge number of people.