Hi Nayeela,
I am currently pregnant with number 2. For me, it was not optional and here's why. I grew up in a big family (we were 5 kids) and I loved it. We didn't always have what we wanted but always what we needed. We had to work hard for what we wanted and it really taught us to be hard-working people who really value the things we have earned. Things are more pricy now but I wouldn't give up one of my siblings for anything.
I think an only child has a very lonely childhood. In reality, as close as you are to your LG, you won't be when she's older. There were things that I would never speak to my mom about but would speak to my sister like boys and silly teenage stuff. Sometimes, what I told her she told my mom and then my mom knew what was going on and could step in if needed. Also, when it came to having a play-mate, my brother and I spent every day together, we were never bored. Plus, having siblings helps develop good personality traits. They usually tease you, so you can handle it when you start school. You have to share and you learn how to live with people which gives you better people skills.
The other thing to think of is when your LG is grown up. Nothing can replace family. My mom died a few years ago and I'm so happy that I have someone to remember her with. Plus my sister and I have had our children together (she's pregnant now too) so we are still sharing things and helping each other out.
So for me, it was more about how much I would be giving to my boy by giving him a sibling. Yes, he won't get as many toys, but honestly, toys get boring - another human being doesn't. Yes, he won't get all my attention but he'll have an adoring younger brother or sister who will probably worship him (as younger siblings usually do) Plus I know that if anything should ever happen to my DH and I, he will always be connected to another person. He will always have a family.
That being said, I think that this is a very personal decision. This is just why I decided to turn our lives upside down (again)