by increasing pressure,boiling point of liquids increase . why ?

For this first understand what is boiling point.

The temperature at which vapour pressure of liquid is equal to the external pressure is called boiling point  at that pressure.

We know that vapour pressure increases with  increase in temperature.

Now if the external pressure increases, vapour pressure also has to increase to become equal to the external pressure. This means that more heat has to be supplied to increase the vapour pressure. Hence the temperature at which external pressure and vapour pressure become equal to each other is higher than the previous one.

Hence. the boiling point will increase.