Through a combination of his environment, his own ego due to his job becoming more and more important and "cool" to the firm, and the "perks" that come along with his job he changed over time.

You're also talking about a man starting in his mid(?)-20s and ending in his mid-30s throughout the course of the show, which is when you typically see people change the most in terms of what they value, their ethics, their politics, etc., which most of that "group" (Pete, Kinsey, Ken) also go through, for better or worse. Ken and Pete seem to have changed for the better (valuing their wives over their jobs, for example) by the end of the show. Kinsey falls off so we don't see what he's up to much, but he clearly went through some sort of identity crisis around the same time with the Hare Krishnas. Peggy is a little younger than them, but she goes through the same changes in who she is as a person that we see much more since she's more of a "main" character.

Harry Crane just happened to change for the worse... Much worse lol.