My mother was one of 6, my father one of 5.
I am the middle child of 3; the 3rd (my younger brother) I suspect was not planned. My older brother is 2 years older than I am; my younger brother 3 years younger.
Personally for me, there are only two things that are really keeping me from wanting 3 kids: The environmental impact, and the fact that my wife only wants 2 (she was one of 2).
My older brother and my younger brother are my best friends outside of my wife. If I need to talk about something, I call them. I can't imagine not having one of them. Additionally, it is a built in support system. Obviously family relationships can tarnish, but in general your siblings are your friends for life. I really saw this when my grandparents were dying. Having the support system and the ability to spread it among different siblings with a dying parent is priceless.
Additionally, my brothers and I are all completely different--and also completely alike. My older brother is brash, aggressive, and also kind hearted and a true people person. I'm overly analytical and introverted. My younger brother is less aggressive than either of us; more analytical than my older brother but less so than me. It has been fun watching him grow. I know that when I got married, I struggled with which one was going to be best man; when my older brother got married, he did too. If my younger brother gets married, I don't know what he's going to do.
It is a special thing, a sibling relationship. It is a good thing. I fully support more love in a family--it doesn't make things any easier, or cheaper, but it may make things more whole.
Other things to note: Being a dad for the 3rd time at age 40 kept my dad young. YMMV.