I haven't seen this movie since the 90's so my question is, why did
Kane retire in the end?
because he had just been down the gullet of an interstellar cockroach, which was one of a hundred memories that he didn't want :-)
"I've SEEN things you people wouldn't believe...."
He was tired and burnt out. I think he just wanted to be done with it.
shareIt was his girlfriend.
When the first encounter with the aliens happened K may have been on the way to see his girlfriend as he was carrying some flowers.
In the picture of the encounter they show him giving the flowers to the alien.
I believe that after the encounter K then joined MIB. I presume they did't wipe his memory at that stage.