Having a second child was important to me so DS had someone else in the world who understood his family life and where he's coming from.
My sister and I aren't amazingly close although we are very fond of each other. We fought a lot as children. But I'm so glad she's there, she understands me and our parents in a way that no one else does and has been supportive at some tough times in my life.
I wanted a sibling for DS so he has immediate family as an adult, other than us, and especially when DP and I get old, to share the burden. I'd also like both my children to have the chance of having nieces and nephews.
Selfishly - when we're old, we'll have more chance of the younger generation being there for us - or even just visiting occasionally! - if we have two children, and twice as many possibilities for grandchildren. My parents and the ILS both have two DC, but only one had kids. They get so much pleasure from the grandchildren. If they'd stopped at one they might have missed out on that.
Also, I wanted DS to have someone to play with, and although they are 4.5 years apart they do play together really nicely sometimes.
DP admitted to me (a bombshell) after we had DD that he hadn't actually wanted a second child, he just went along with the idea. However he's so glad we did now, he adores DD and she adores him.