Why do squad rotation players get upset with lack of first team football?

It's mid season and I've got a few players on squad rotation status complaining about lack of first team football. The thing is some of them are fairly old with low stamina and can't run out whole games anyway, or are simply not quite as good as my first teamers. Also its not like I've just stuck them in the reserves, I do rotate my squad a bit due to a hectic fixture, my backup winger has had 6 starts 4 sub apps whilst the backup AMC, who is great technically but with 3(!) for stamina simply can't run out a game, has had 19 sub apps, surely that's enough game time to justify their squad status?

Does anyone know how this will affect squad harmony? Right now its still OK, but I don't want a bunch of whiners dragging down squad harmony when they clearly agreed to be squad players when they signed on the dotted line!

Rotating players does make the squad lack harmony but don't be worried about players getting unhappy. Say they will be playing next week to the media and all will be forgotten. You don't even have to keep your promise