1) this is Watson-Crick pairing, but actually, they do not have to pair in this manner (especially in RNA)
2) they form hydrogen-bridges, G-C three, A-T two 😉 Other pairing is less favorable and leads to irregularities
-Hydrogen bonding and also hydrophobic interactions between bases. The nitrogenous bases pair mainly by hydrogen bonding but since DNA is in a high water concentration in its native state the bases also show a hydrophobic tendency and move closer to eachother and away from the solution, which causes you to have a B form DNA strand.
If you put DNA in vitro with a low water concentration, you would witness the bases would stretch further apart. This is due to being in a lower water concentration thus, the bases are "less water fearing" thus the bases are further apart from eachother.
But the major force would be the hydrogen bonding, but it is also good to know the hydrophobic reactions that go on as well.