Plants typically drop leaves due to a lack of sunlight, under watering, overwatering, or watering with water that is too hot or cold.
Lack of Sunlight -
If the plant's leaves are beginning to turn yellow before falling off, it is probably due to a lack of sunlight. Find out more here.
Under Watering -
If the plant's leaves are turning brown and crunchy before falling off, it is probably due to under watering. If the soil becomes too dry between waterings (even just once) this can cause the leaf tips to brown, and leaves to fall off. This can also cause perfectly healthy looking leaves to drop. Find out more here.
If the leaves are very droopy, or turning yellow and brown before falling off, it may be due to overwatering. Find out more here.
Watering with Water that is Too Hot or Cold -
If the plant suddenly begins to drop healthy looking leaves, it could be due to watering with water that is too hot or too cold. Watering with water that is room temperature is ideal.