You have to watch out though. Kissing can expose you to lots of potential dangers, one of which being Epstein-Barr virus (EBV). This can cause infectious mononucleosis or glandular fever.
On the other hand, kissing could be useful in gaining adaptive immunity against EBV, but you run the risk of getting ill with glandular fever. Still, I think the positives outweigh the negatives!
Glandular fever is a terrible disease to have. It's a risk that's always there!