Dropping its leaves is one way a plant show its stress. Too much or too little water causes the leaves to dry and drop. If the soil in your pot feels cool, the plant probably has enough water. If algae or moss is growing on the sides of the pot, you're keeping it too wet. Some plants, particularly those that produce flowers, need plenty of light.
Insufficient light can cause leaf drop.
And of course, a plant will drop its leaves during the natural aging process. If none of these conditions seem to fit and you burn propane gas, call the gas company right away. Houseplants can be very sensitive to gas leaks and warn you by dropping their leaves. Those little sticky white puffs that look like dabs of cotton are probably mealybugs. The actual insect hides beneath this waxy, cottony covering and sucks the juices from the leaves.