A and T pair with each other because the adenosine molecule structure has one proton acceptor, a nitrogen, and one proton donor, a hydrogen bonded to a nitrogen; the thymine molecule has one proton acceptor, an amide functional group, and a proton donor, a hydrogen bonded to a nitrogen. The proton donors and acceptors on the adenosine and thymine need to form a covalent bond with each other.
An adenosine or a thymine cannot bond with a cytosine or a guanine because cytosine and guanine because cytosine and guanine has two proton acceptors and one proton donor, and two proton donors and one proton acceptor, respectively.