I believe Clemenza and Tessio always liked Michael, in an avuncular kind of way. In fact, they had no reason to dislike him because he had never been part of the Family business and therefore had no conflicts with them, did not pose a threat to them, etc. Before Michael volunteered to get involved by killing Sollozzo and McCluskey, Sonny, Tom and the two caporegimes wanted to keep Michael away from direct action because they knew that Vito wanted Michael to be a legitimate pezzanovante. As Sonny said in the film, "Madon', Pop would have my head..." if he got Michael involved.