What do you look for in a Graphic T?

I feel like I have been over thinking Graphic T's. The point of them is they have more personality and interest than a plain T. However I always worry about them having too much personality and then being "defined" by the T as though it has to be a primary interest. Recently I have been buying them as keepsakes from vacation which has enough "story" behind it that I feel justified in wearing it. However I want to expand my collection so as to have more options. I work an office job which has a business casual dress code that is fairly conservative. As a result I sometimes feel my outfits are overly formal for the situation.

When I shop online most T's seem to be sports related (not a fan despite being in a huge sports city) or music related which I might like the T or the band but I feel like buying a Nirvana T means I should be listening to their studio albums on the regular. Perhaps this is over thinking it but I view fashion as presenting a certain version of yourself. Others perceive you differently depending on what you wear. Therefore I'm curious how others approach buying Graphic T's. Do you buy what's trendy? Buy purely on aesthetics? Pick things up as you venture through life? Steal them from your lovers?