Your baby makes a "sleep association," wherein he associates certain things with falling asleep and believes he needs these things to fall asleep. My baby, Coleton, spent much time in his early months in my arms, his little head bobbing to the tune of my computer keyboard. From the very moment he was born, he slept beside me, nursing to sleep for every nap and every bedtime. By the time I looked up, he was 12 months old, firmly and totally entrenched in a breastfeeding-to-sleep association.
Your baby, like my Coleton, has learned to associate sucking (having your nipple or his bottle or pacifier in his mouth) with sleeping. I have heard a number of sleep experts refer to this as a "negative sleep association." I certainly disagree, and so would my baby! It is probably the most positive, natural, pleasant sleep association a baby can have. The problem with this association is not the association itself, but our busy lives. If you had nothing whatsoever to do besides take care of your baby, this would be a very pleasant way to pass your days and nights until he naturally outgrew the need. After all, this is natural. You may not even see this as a problem, in which case it is not. It's all a matter of your perception and your personal needs.
However, in our world, few parents have the luxury of putting everything else on hold until their baby gets older. With this in mind, I will give you a number of ideas so that you can gradually, and lovingly, help your baby learn to fall asleep without this very powerful sleep aid.
To take the steps to change your baby's sleep association, you must complicate night wakings for a while, but in the long run you can wean your baby from using his pacifier, bottle, or your breast as his only nighttime association. In other words, be prepared to disrupt your own nights for a while to make some important, worthwhile long-term changes.