they all get gifted too early. a full england cap doesn't mean much anymore - they'll have a friendly and you get about 7 or 8 making their debut - then at the same time they're getting rewarded at their clubs e.g. 40k a week after about 3 or 4 months of doing something good.
of course they're going to get big headed - they're all going to be a tad bit snobby being asked to come back down to the U21's. you should only really get a select few who have to go back down to the U21's after making a handful of full international apps - i.e. the ones who have proved they're not ready to make the step up.
i always use ozil as an example - such a vastly talented player and still so young - yet he really can't be arsed 80% of the time. he's nearly won it all already and he knows he can stroll around the pitch week in week out and will still earn about 80k - would you really bust a gut in his position if you're not that kind of person?