Why does my baby keep doing this?

Either he is having a hard time holding on to the pacifier on his own. You could try a different type of pacifier. And another thing, even at 2 months old babies are very intelligent. Let him cry it out for a little while before rushing to put it back in his mouth every time. He knows that if he spits it out, mommy is going to run right back to put it back in his mouth. Try one or both of the things i suggested, i hope it helps.