In living systems molecules involved in hydrogen bonding almost always contain either oxygen or nitrogen or both. how do you explain this phenomenon?

This shows that both the nitrogen and oxygen are the elements that exhibit extremely high attractions for their electrons.

A hydrogen bond refers to a partially electrostatic attraction between a hydrogen that is bound with a more electronegative atom, like oxygen or nitrogen, and another neighboring atom bearing a lone pair of electron.

More the electronegativity of the hydrogen bond acceptor more will be the hydrogen bond strength. The hydrogen bonds are accountable for holding together proteins, DNA, and other macromolecules.