why do we call lone *pairs*?
We call them "pairs" because there are two electrons in each grouping. Why are there two instead of three or just one?

Spin and orbital energies.

One electron up and one electron down, in order to have all electrons in the lowest energy orbital available. Having them separate is a vastly higher energy state for the particles as a whole.

If you have not yet studied orbital theory - your question will be answered in much more depth when you do.