You've probably felt love before, otherwise you wouldn't be here. Ask yourself what makes you a leftist.

I think the most committed and righteous of us feel an anger or rage when they see injustice or suffering directed at others. This is but a secondary emotion. It's an articulation of a love they have for others just because they are people like them in circumstances outside of their control. For me romantic love is an individualised highly focused form of that emotion that then triggers an urge to unite with the targets of that focus, be it in time, place, life trajectory and other things. Sexual attraction is then an additive component to that. Sometimes it's there first and love develops after, sometimes it develops over times. Sometimes it fades after, sometimes it doesn't. I think the first sign that someone is in romantic love with someone else, is when they spent most of their waking hours with them, physically or just thinking about them. This is just the infatuation phase and generally fades and develops into something else.

These are just my understandings and not drawn from personal experience so keep in mind it's more probable than not that I'm just talking out of my ass. I hope my worthless musings can inspire you to find a thread you can pick up and come to your own conclusions though.