Does it? If you're defending, it's presumably from an attack, which you are thereby opposing.

I mean, imagine if Russia and the US announce tomorrow that they formed a military alliance - it's just defensive, don't worry about it. Also, UKIP, AFD, and FN won, and the UK, Germany, and France are joining, too. No, the rest of NATO can't join, but it's just defensive, don't worry about it. Also, Taiwan can't join, but China might. Don't worry about it.

I get that exclusion doesn't necessarily have to mean hostility, but I don't think it's a stretch to see a power block forming that deliberately excludes you and be concerned. Now, I could think of a few ways of dealing with that concern that don't involve, yknow, going to war, but I'm not the guy running things.