Question

why boiling point increases with pressure ??

Answer 1

The vapour pressure (is a physical property of liquids) gets higher and by consequence so does the boiling point. This is a property of all liquids and also water. See : http://en.wikipedia.org/wiki/Vapor_pressure

Answer 2

You can see the process like this: with higher pressure in a gaseous environment (like in a pressure cooking pot), there are more collisions of the molecules in the gas phase, and so more molecules will be bounced back into liquid phase and give up beeing gaseous. More energy is needed to become gaseous, and thus the temperature needs to be higher to send molecules in the gas phase. The boiling point move to higher temperature (pressure cookers reach temperatures of water boiling point of 120°Celcius). Cooking in the Himalayas at high altitude will be at lower temperature, for ex 80°Celcius or so. There are charts available for vapor pressure for most materials.
This is terrain of Thermodynamics.

Answer 3

Boiling water is water molecules with so much energy that the weak chemical bonds holding them together are broken. The energy in a molecule is in the form of motion. Water molecules are always jittering around randomly, even in ice. As long as they're above absolute zero temperature, the atoms making-up water molecules ( or anything else ) are jiggling around. But the electrical attraction holding atoms together ( chemical bonds ) is broken only when the jiggling-around is so great that the force holding the atoms together is exceeded by the force pushing them apart ( by the heat- or random motion of the atoms ).
Think of a bucket of velcro balls. They'll stick together as long as no-one is shaking the bucket much. But if you could get the bucket of velcro balls to bounce around enough the balls would unstick, especially on the surface of the bucket. The balls in the middle and bottom of the bucket might temporarily unstick but they would likely bounce off other balls and re-stick to each other. This is similar to water.
The surface of a boiling pot of water has H2O molecules shaking around enough to go flying in all directions. The atoms that go up away from the pot are evaporating or boiling off. Other atoms go flying down back into the pot and might re-stick to other water molecules if they settle down ( lose energy ) a bit.
So to answer your question: Why does boiling point increase with pressure?
Because pressure is something holding the molecules together. Like swimming to the bottom of a pool. The water above holds you down and makes it harder to escape.
Water molecules are held together by the pressure from above and thus it takes even more energy ( heat ) to jiggle the molecules enough to break them free of each other.
If you increase the pressure, you're increasing the force pushing things into each other. Like squeezing more people into a small room. The more people in the room, the higher the pressure and the harder for people up against each other to get away from each other. The fewer people in the room the less they'll be stuck to each other.

Answer 4

Because pressure is something holding the molecules together. Like swimming to the bottom of a pool. The water above holds you down and makes it harder to escape.
=> sinking is not caused by the water above holding one down, but by compressing the lungs which causes the body to have lower volume and thus less displacement of water and thus lower upward force (Archimedes). This is indeed very dangerous for unexperienced divers "apneu".