Why are my relationships with vassals and allies declining?

[gribol]

Playing two independent countries and in two separate games, I had a similar situation.
At a certain point, relations with vassals and allies start to drop significantly every turn for me until they all revolt. What is the reason for this? Do I have too many provinces? Do I have too many vassals? Is this described somewhere? Why does this happen?
It somehow prevents any development and expansion.
EDIT: Thanks to Surt i have seen the "overindulging" message in diplo. Whats thats means? How to deal with that?

[EHamilton]

One reason this can happen is due to secondary diplomacy effects. If you give a gift to one faction, all that faction's enemies will become angry with you.

One situation where this happens is when you were at war with a faction, then made peace. Now you want to give them lots of gifts, so they will be your friend instead. But all you vassals still hate them, as a lingering result of being called into your recent war against them! Every gift you give causes you vassals to get angry with you. If you have a lot of vassals, you'll be gaining reputation with one target but losing it with a dozen other factions. A very bad trade-off.

[gribol]

Oh, that can be my problem, I was bibing them very much.
Thanks a lot!