It's possibly worth pointing out in later life he seemed happier about it -his second autobiography was called "I am Spock" https://en.wikipedia.org/wiki/I_Am_Spock
But on the other hand, that might just be an oldish man not really feeling comfortable playing an immature character (that's part of Han's charm, really).
I'd wager his dislike of the Han Solo character had more to do with working with George Lucas.