HotI, I truely believe it is personality and genetics to a degree. I was by all accounts a terrible sleeper, didn't go to bed willingly or to sleep easily.
DD1 - is still a very difficult sleeper. She slept ok as a NB, she was jaundiced and I had to wake her every 3 hours to feed for the first few weeks. She did get longer stretches but when she was 9 months old she went to childcare and had lots of continuous ear infections and then teething. This involved lots night waking and disrupted sleep for a good 8months. We never really got a routine back. She is 6 and still frequently wakes at night, she gets quite anxious about being alone and is a bit of a worrier. I try to make sure we have a calm time before bed and I chat and try to discuss any concerns she may have. Sometimes this backfires as she gets caught up in thoughts. We do talk about trying to relax our minds so we can sleep - we also had a discussion that sometimes we can't just got straight to sleep and that is ok . This has made a massive difference as she was panicking about not being asleep ITMS? She is also able to come into us if she wakes at night, not ideal but really I have a seriously long term sleep deprivation issue and we have just decided to deal with it in as less traumatic way for us all. Oh and we bought a king size bed - it has been the best thing ever. Not sure why have a 3 bedroom house as we do not need it! Things were pretty dire when DD2 was born and I got by on probably 4-5 hours of broken sleep for about 18months. It has been hard but we have also put pressure on ourselves to 'fix' it when really there was little I was happy to do that would have fixed it.
DD2 has been a more solid sleeper (and yet she was cosleeping/demand fed as a baby) but is ultra competitive with DD1/super clingy with me atm and so is waking screaming for me at about 3.30am. That's ok and instead of trying to fight it I just jump in with her. We are all getting much more sleep than we have been and slowly catching up.