I have to agree with J G. He realizes (believe it or not!) that when he cried harder and for just a little bit longer than the time before, you came in. He won. And so tonight, he'll cry just a little harder and little longer still until you cave again... so on & so on, making these fits longer and longer each night. Which isn't fair for either of you.
I went through this with my son at 8 months old. There was nothing that was hurting him or making him uncomfortable, he just wanted to be held to fall asleep (which is really sweet until he's been doing this every hour on the hour EVERY night for eight months straight! LOL) Finally, after trying everything else, my husband said we just HAD to let him cry it out. I think I may have cried harder than my son that night! But, it was only that one time and he slept through the night from then on. It really was difficult and it made me feel like such a horrible mom but after it was over, baby & I (and the whole house!) slept better and felt much better during the day! Good luck to you!