I just prefer trusting my own instincts I guess, I've done this for years and I like to think I have a good eye for design. Of course no one is perfect and things could always be improved, but I think a part of me just wants myself to be the only influence on my designs, for better or worse.
Edit: In fact, the more I think about it, that's almost entirely the reason I don't like collaborative design,
I want the credit for a design lol. And in the case of a design contest rather than work for a client or something, to me personally, collaborating almost feels a bit like cheating because I was not the sole creator in a sense, depending on how much other people influenced a design. This is just how I feel about it for my own personal work, don't get me wrong - I would not begrudge anyone for collaborating on designs or think less of anyone's designs because they were the work of more than one brain. I don't think there is anything inherently wrong with collaborative design and in a lot of cases it's probably ideal for getting to the best design for a given purpose, but I've just always wanted to be able to say this is entirely my creation with no outside help. I'm stubborn