There are several reasons as to why leakage happens, the most common are as follows -
1. Oil Filter: It can wear out with time, aligned improperly or be loose. Every time the oil is changed, the filter should be checked for proper fitting.
2. Oil Drain Plug: This can be worn out, misaligned, or loose as well. It can be found underneath your car. If this is the cause of leakage, there will be fresh oil around it or dripping on the side of it.
3. Oil Filler Cap: If not closed properly, or broken, pressure created by the engine can cause the oil to spill out.
4. The Oil Pan: It can be damaged when run over debris on the road. This causes dents which compromises the oil drain plug causing leakage.
5. The Valve Gasket: This is a seal that joins the two main metal parts of the engine. If your car is old, or hasn't been serviced in a while, then this may be the cause of leakage. It gets damaged due to the buildup of sludge causing too much pressure on it.
Do be proactive and check for minor leaks as it can lead to severe engine problems in the long run.