It's all explained in Morgoth's Ring. Elves give part of their spirit to their children. Which is why Feanor's mother Miriel was absolutely knackered after giving birth to such strong, energetic spirit. So after a few children (Feanor's 7 was the greatest number in history I believe) they lose enthusiasm for more so they devote their attentions to other Elvish pastimes like singing and looking down on dwarves.
