i irrationally hate the actor that played the mother from How I Met Your Mother. Maybe i just hate that doe eyes look she seems to be inflicted with. And she seems to always be playing some sad sack in a bad relationship so it might just be type casting too. Plus the end of HIMYM was so fuckin bad so that might not help lol.
Of all the responses, this one has shocked me. The dude can actually act. So yeah, purely irrational.