There are a number of reasons a trap looses its seal (depth of water).
1. The trap nut on the trap can be broken allowing the water to drip out under the sink.
2. The trap could be improperly vented. The arm from the trap should run horizontally directly into the wall at no more than 1/4" per foot, or the trap will syphon the water out every time it is used.
3. The trap could be unvented. Again same answer as above.
4. Wrag fibers could be hanging over the bend of the trap into the trap arm, allowing the trap to be drained dry by the process of osmossis.
5. The line could be clogged or the vent could be clogged and the pressure is blowing sewer gas out of the other trap. (Each gallon of water pushed down the drain, pushes a gallon of air ahead of it. Vents relieve the pressure.).