They had some rough times before (season 1 comes to mind). Every season we've seen fights between them. Some friends are like that for real : they keep on arguing, sometimes very harshly, but never forget they're friends anyway.

However, Sheldon being who he is, Leonard is always the one who must be tolerant and forgiving. Now, having a glimpse of how life could be without permanently living with Sheldon must be quite tempting. Hence his reactions.

Once Leonard is done with Priya, I think he'll find his friendship with Sheldon quite comforting... even if somewhat weird, as usual.