It's probably "pot-bound" among other possible problems. Gently excise it from it's pot and examine the root ball. If much root tailings visible on the outside of the ball, then time to transfer to a larger pot. In the future, systematic pruning will eliminate the need for transplanting.
Other solutions might include water, nutrient, and sunlight ramifications.
For starters, you should know why the leaves are falling off in the first place so that you can take the appropriate step. In most cases, leaves fall off due to over watering or bug infestation. In case of the latter, get some q-tips and some rubbing alcohol to treat them and on over watering, just make sure you allow the soil to dry-out before you water again.