Sazzlehopes - you asked about DS's age/gap on the other thread.
DS was 4 in October and we did want a big age gap (even though I am a bit older now, I was 38 in July). For a couple of reasons, I wanted to be able to have that 'Me & You' time with DS for a longer time - Perfect Firstborn love-in I suppose 
I wasn't absolutely sure I wanted another one until this summer either, I had a tough pregnancy and a big surgery linked to that when DS was 1. Took a long time to recover mentally.
Lastly, DS will start school in Sept (he's at Montessori now) and that will hopefully make things a tiny bit easier on everyone with a new LO. NewLO gets 'Me & You' time while DS is at school and DS can have his 'Me & You' time when he gets home .. and I might still have time to ride my horses!
But basically, the main reason is that I just wasn't really, really, sure until this summer. Then all of a sudden, broody!