I found the line in my text book. But why is it so?

I think you're talking about enthalpy of formation for a substance that is synthesized from its constituent elements. The less positive (or more negative) the enthalpy of formation for a substance is, the more stable it is because it takes more energy to disassociate it back into its constituent elements.

See, there is a general phrase used in chemistry that " lesser the energy of the substance more stable it is".
The reason for this is that, substances with lesser energy will have very little tendency to react.
For more clarification, post your thoughts.

What ebaines said seemed very clear to me. I have understood it. And I have also clicked on 'helpful'