It's 100% personality, without a doubt. Having your kids v close together assuming they will get on because of the small age difference is a huge gamble.
DH and his brother are 18ms apart and haven't got on since day one. They didn't 'share the same interests' as they were chalk and cheese!
There are 5.5yrs between dd and ds2 and I really worried that the age gap was too big as ds1 in between them is severely autistic, so they will only really have each other to interact with and it felt particularly important that they got on and could support each other with the challenges a disabled sibling brings.
So far (ds2 is now 4) I really needn't have worried. Dd is v loving and maternal, and adores being a big sister, teaching her little bro things and teaching him things and for his part ds2 worships his big sister and has become accomplished at socialising with older women as she has her friends round all the time and, as with dd, they are mature enough now to pander to him and include him a bit rather than bash him
and scream at him.
There is no competition between them because they are at different stages, and their personalities are incredibly similar so they have a great laugh together most days.
We feel very, very lucky it worked out like this. Touch wood!