The Ennui of the Elves

[Bombadil]

Elves began it, of course, waking trees up and teaching them to speak and learning their tree-talk. They always wished to talk to everything, the old Elves did.

I believe it was all just a matter of time and events. The trees were there before the elves, and before the elves knew fear, hatred and mistrust I'm sure they were glad to be open to any other creature or thing besides themselves. But as time moved on they were hit with the constant destruction of Melkor, the kin-slaying of the Noldor, the animosity between different elven races created by Fëanor's actions, the mixed emotions towards the second born, the betrayal at the Nirnaeth Arnoediad...

The list goes on and on, and for an immortal race these kind of events won't be dead and forgotten in memory, they will live on with the very fire of anger that was kindled the first time, or the lament will stay just as sorrowful. That would be my reasoning, that the world they lived in had shown them reasons not to interact with the outside world. Perhaps it all started in Doriath with the need for a hidden kingdom?