I think it will depend on what your natural size was when you started.
A guy that is a natural 5 inches might tend to say Fear of sexual failure with a partner or Insecurity in the shower/locker room.
Whereas a guy with a natural 7 inches might tend to say Vanity and ego.
However those are just general rules.
A natural 5 or 7 may say anything at all because the guy that is 5 is confident with his size but wants more anyway, whereas the guy who is 7 is not confident with his size and wants more to improve his confidence.
Psychological factors may play a role.
For me it was just wanting to push women to the brink of insanity when it comes to sexual pleasure.
I LOVE WATCHING THEM GO NUTS!!!