People want to be loved. No one cares about rep in and of itself. People just seek the approval of others. Neg reps hurt, I'll admit it, even if it's just a little bit because you don't think much of the person giving it to you it still doesn't feel good. When I make a post and feel like it was clever or well thought-out or informative it absolutely feels good to get a pos rep for it. It's positive affirmation.
The little number under my name means a lot less, but during a dry spell I can still look at it and say "At one time at least for a moment someone valued what I'm doing here."