One of the criticisms that I read of Shatner was that at the end of the day, instead of hanging out with his fellow actors, he would leave the studio and spend time with his established friends and his family. He treated his fellow actors as co-workers and not his new best friends.
I thought that as well, until I read Koenig's book about the making of TMP. There are photos of the cast and crew playing softball and Shatner is there. I was surprised to see that.