They think you are a weakling, which is true, you let them attack you without fear of retribution. I say you need a punishment expedition. March to berlin and sack it, march to london, sack it too, and go home. You will retreat within the time pope gives you, and you looted their capitals, their infrastructure is wasted and you swim in cash. Also, you get respect from them.
Or, you could simply conquer few countries, to permanently shut them up.

The thing is, I make nice kingdom, and everyone is my pals. Then they stop liking me cause I have so nice empire, and I must go and conquer them to stop them from attacking me. And my new neighbours respect me. One day they turn against me, and I have to silence them aswell. Get new neighbours again.
It's a vicious cycle. You either conquer or you will be conquered. If not by your neighbours, maybe by some distant foreign power that takes your weak neighbours. And they will crush you with their overwhelming power. Grind you. That's why expansion is vital. You need to be powerful to fight off powerful enemies.