Our son, who will be four in February, is an only child. I've read books from the public library about raising an only and found them helpful- that may help you too. Both my husband and I agree that one is just right for us. Like any situation, there are costs and benefits to being an only.
Here's what I've read and taken away on the subject:
1. Onlys need socialization and playdates. As he grows, he'll benefit from close friendships not just with a peer, but your being close with another family. Particularly helpful is close relationships with out 'only' families. Try and create sibling-like experiences, sleepovers, take his friend on vacation with you, etc. I read this is particularly helpful for holidays when families tend to turn inward.
2. Without sibs, Cruise gets all our parenting attention- good and bad. I'm guesing there are times he feels like a specimen under a microscope- as we are able to focus entirley on him. I know this is common with first children and when there's no sib to demand you divide your attention... So, I try hard not hop on every little misstep. On the bright side I know he relishes our undivided attention during all the fun stuff and is having a great time.
3. All personalities are different, Cruise has always been great at sharing with other kids- lucky us, but I've had to work at developing his independent play. I started this around 18 months and just kept at it, daily insisting he 'find something to do'. Now he's very good at it. Onlys will spend more time alone than with sibs so it's important they learn to entertain themselves and enjoy their own company- again, helpful life skill here for later in life.
What else...
4. There's a terrific section in a child rearing book called, "Your Baby and Child" by Penelope Leach, about the whole "Does only make lonely" issue (they say no). I found it sensible, well-researched and extremely reassuring. It doesn't stop with saying, "they won't be worse off" but add "they are likely better off in these ways". Check it out if you haven't.
5. A sib doesn't guarentee a bond. Some sibs don't like each other and never really do (you may know some yourself). Just different personalities or family dynamics that never get out of the way. The truth is, a sib can be a short term or even lifelong irritant more than a comfort. This fact is an uncomfortable reality it seems few people want to admit, even to themselves.
5. When we were sure of our decision to have one, I grieved my son's lack siblings (we both have them) and the possible extra burden of having to care/ worry about aging parents with no sibs to spread it around. I still grieve about these things and probably I always will. But that grief is smaller than our contentment, that we did what's right for us.
We know our limits for the amount of emotional, intellectual and financial support we can provide offer and feel great that we have created a pretty great environment for our one child.