The difference was that when mexico controlled those states when no one lived there. The united states came in and populated the western part ofthe country, before that it was territory as worthwhile as Alaska. That settled who controlled the area.
Kashmir is different though since it could be like nepal or bhutan where it's in the norhern part of the sub continent but influenced by a few powers. I know pakistan was created by separatists, but that is a decision made in the past and people have to deal with the situation now.
Believe me i would like to see an agreement over the demarcation of kashmir and let the problem rest, but that doesn't seem to be happening anytime soon. Rather than have another split in a province like Punjab, perhaps leaving it intact and independent is the wiser choice. After all what would either country gain from controlling the region? Nothing more then if they didn't control it.
It would be nice to think that the subcontinent can get along and that kashmir can be settled, but people just have problems letting things go in these sort of situations, it's sad but there are other solutions.