Among elves, the act of cutting hair is associated with disgrace, defeat or debt to another individual. 

It hampers their ability to sense subtle aether fluctuations around them and can lessen the otherwordly knowing elves are widely known for. It's not common for elves to willingly cut their hair.

It is more common for conflicts between Elven forces to result in the defeated commander having his hair cut or cutting it himself with a sword. 

Sometimes an elf will cut some or all of their hair as a way to make a vow or important promise. This can be lost on other races, but among their own people it's taken quite seriously.