Janni has made a good point there. I have two DS (2.5 and 4.5) and it's only recently that I've realised that actually I don't have to make myself at their beck and call all the bloody time to be a good mother to them. They are quite capable of playing on their own for periods, and I am now equally quite capable of saying "No, I do NOT have to come there straight away to fix the train track actually - YOU have a go at fixing it whilst I finish loading the washing machine, and if you haven't managed to fix it by then, THEN I will come and fix it for you." Why did it take me 4 years to realise that no, actually, I don't have to drop everything (literally, if I'm loading the washing machine LOL), and go and sort out a non-emergency situation. It is also teaching them patience and initiative whilst they are waiting for me to finish this, that or the other.
Also, when DS2 was having a big nap after lunch when he was a baby, I was nearly going insane with the conscious effort of rushing my lunch down so that I could give DS1 some 1-1 attention the whole time the baby was asleep. After months of nearly losing it by the end of each day through sheer tiredness, I suddenly thought, "Today, I am going to start having MUMMY'S REST TIME straight after lunch, and DS1 can occupy himself for half an hour while I have a cup of tea, put my feet up and read the newspaper." He was 3 at the time, and it took 3 days for it to sink in, and from then on he would sit patiently and look through a magazine or watch a DVD and know that I'd be refreshed and ready to play with him after the half an hour.
Sometimes us mums get caught up in trying to be bloody Mary Poppins 24/7 that we end up nearly giving ourselves a nervous breakdown. (I'm sure Mary Poppins was on something to be that cheerily available the whole time, or maybe it was just because she got a regular sweep up her chimney. )
To the other OP, you might find this thread http://www.mumsnet.com/Talk/8/613387 from the other day helpful.
As for your question about how come people have more than 1 child, the answer is that they have the easy child first, think that this parenting lark is a doddle, then naiively have another thinking it will have exactly the same type of personality as the first. BIG MISTAKE!