Overwatering — the most common cause
Soft yellow leaves paired with soil that stays wet for several days most often signal overwatering. Roots, deprived of oxygen, begin to suffer. Check the pot's drainage and space out waterings.
Underwatering
Conversely, yellowing combined with dry, brittle leaves and completely dried-out soil signals underwatering. Resume regular watering, without overcorrecting.
Insufficient light
Gradual, widespread yellowing, especially on the oldest leaves, may simply mean the plant isn't getting enough light to support all its foliage. Move it closer to a brighter light source.
Nutrient deficiency
If the soil hasn't been refreshed or fertilized in a long time, yellowing between the veins (with veins staying green) can indicate a nitrogen or iron deficiency. Fertilizing appropriately during the growing season can resolve this.
Natural aging
One or two older leaves, usually the lowest ones, turning yellow and dropping is part of the plant's normal life cycle — no cause for concern if the rest of the foliage stays healthy.