Well, breasts are incredibly warm, soft, and comforting. When I think of my girlfriend's breasts, I immediately think of comfort. More than likely, if he's sucking in a calm, non-sexual way while cuddling, then he is simply enjoying the warmth, comfort, safety, and security of being at your breast.
The experience can be very soothing and relaxing for a man. After a rough, stressful day at work, cuddling up against a girlfriend's breasts and sucking her nipples can pretty much cure a bad day, even if it was the worst day of our life.
If you're comfortable with it, just close your eyes, cradle him close, and enjoy the intimacy. There really is nothing else like it in the world. This type of intimacy can become even more intense. For further reading, check out http://www.landmilkhoney.com/
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