I seed because of the bakabt's ratio rule, if it wasn't for that ratio I wouldn't even think about seeding.
Yes, I'm a selfish person, I close my torrents after downloading BUT I do seed occasionally: after downloading a new episode, a request or when I just feel like it.
I have unlimited upload, with 250gb monthly download cap. When I upload, it slows my net to dial-up speed so I really hate seeding.
tl:dr I only seed when its necessary, or someone's requesting it.