Crowded teeth, the opposite of spacing is caused where there just isn't enough space in the mouth for all those teeth.
Crowding just gets worse over time as one tooth pushes over the other leading to overlapping teeth. Crowded teeth are harder to clean then straight teeth, which will lead to cavities and tooth decay. Crowding can often be correct by expansion so as to avoid tooth removal.
Crowded teeth are harder to clean than right or aligned teeth, making them more vulnerable to tooth decay or decalcification. Crowding can often be corrected by expansion, without the need to extract teeth.