Well, it's hard to put into words, but I'll give it a go. For the gymnast that has been on the team maybe a year or two, it's not such a huge deal, in my opinion, especially if they are young or on the lower compulsory levels. They are just starting out, they don't know all that's involved, etc. Its more about the ones that have been in a gym on a team for many years that gets such a strong reaction to leaving or looking around. Or when it's one of the gym's best gymnasts, that really hurts (feelings). By then it's more like a family member, someone you've grown to love, you enjoy each other and depend on each other. It's the parents as much as the child at that point. We just had two girls leave to try a lesser (yes, I'll say it!!) gym, and I'm still waiting for the one mom to tell me--we've been together for 6 years, we've spent most distance meets together, or at least rode together. I feel hurt that she'd been planning to leave all along, and didn't even tell me the night they told the owner.
Its like a pp said, after years of training and pouring all your knowledge and skill that you can into a gymnast, for them to just leave---its hard to deal with. A new gym just opened up near ours, and has rapidly expanded from just classes, to level 4 team six months later, to renting a much larger facility 9 months after opening, to now building an even bigger facility, and they haven't been open for a year. I don't know how they are expanding so quickly, since they are supposedly non-profit and all, and supposedly don't care about scores even though they are planning on having more teams. And they are getting a lot of our class kids, which I guess is normal since people love to check out new stuff, but they are also getting some of our team kids. Their only team has 5 out of 6 kids that left our team for theirs in the fall in one group--don't tell me that was a coincidence. And now they have some of our coaches listed as their coaches???
So you see, that is why there is often a strong reaction, because it feels like betrayal, even if it's not intended that way. And, it's sad--its sad to lose your friends and teammates that you love and care so much for, and only really get to see at gym because of distance and different schools, etc. And then you see them at a meet wearing different colors, with different teammates, and you miss them all over again, and it's very awkward seeing them and talking to them.