I think having the extra detail helped a bit (for me). The first post sounded like you were talking about every situation, the second post sounds like this has only happened with 2 (possibly 3) kids. Considering that most of our kids have opportunities to build friendships with quite a few other children, it's not surprising that one or two here or there aren't going to mesh. The competition upset does happen - when I wrote my original reply what I'd thought I understood from the first post was that your ds was having this issue with most of the children he had contact with, not just a few.
The piano teacher chart thing - I wouldn't like that *at all*. I suppose it might increase practice time, but is that really the point of piano lessons? To practice more than the other kids? What about learning music for the sake of learning? It's possible that the relationship change with the lego-child is due to something that's happened with seeing the electronic practice reports, but it's also possible something else is up. I might ask the mom that you've noticed something seems to be different and you've wondered about it (if you know her well enough to ask). I also think the whole idea of practicing basketball on purpose to one-up another child is a bit odd... a part of me wonders is it possible the parent was joking about it?
My kids do know kids who are sensitive about winning/etc - but there aren't a huge number of them in the circle of kids they know. If it's just a few kids here and there I wouldn't think any more about any of it, just stick with playdates with the kids your ds wants to be friends with. If it's turning out that these things seem to be happening frequently, I'd look a little bit deeper to see if it's more than just scoring more points, but possibly behaviors in that situation that the other child is reacting to rather than the actual score.
FWIW, all three of my kids (regardless of personality) have had changes in friendships over the years - I think that's quite normal as kids grow and start to really get to know themselves and define themselves.
Best wishes,
polarbear
