When I was younger, the rules seemed simple. You greeted your relatives with a kiss, your friends with hug and your girlfriend/wife/whatever with a snog. Now I'm noticing that an increasing number of my female friends want kisses!
Could it be that my charisma is just in better shape than it used to be? Nice idea but perhaps not.
Maybe there's a deep psycho-sexual thing. Older people have enough sexual experience one way or another to be totally blasé about possible connotations of a kiss. This is quite a good explanation (especially since it grosses out younger readers who would prefer to believe that old people know nothing about sex - still less have it.)
No, it's easier than that.
As you grow older, you and your friends are more likely to have acquired bad backs.
A kiss is just safer than a hug.