loads of reasons, unaccustomed exercise, spending too long standing still at one time, poor circulation causing the blood to pool in the legs, mild heart failure causing the legs to fill with fluid, a thrombosis, muscle damage, viral illness, varicose veins to name but a few. Treatment depends on cause.
I've had aching legs all my life, as a child i was told it was growing pains - i think it is poor circulation, mine gets much worse after drinking lots of alcohol. Probably nothing too much to worry about, i think wolfgang's being a bit alarmist!