As a DIL with a good relationship with my MIL, I think the key driver of any unhappiness on that side comes down to expectations.
She has always had these dreams of how things will be and look, and they usually don't take the other people into account much. Her ideal world would be that her sons live on her road with her best friends and we all went on holiday together and all holiday occasions were celebrated together.
(It's worth saying that my husband cautioned me that this was the case and didn't want to live near her as he didn't want this - they actually barely saw FIL's side of the family, and all FIL's landmark occasions revolved around her family).
On the other hand my parents are welcoming, but place no expectation on how we visit, and have lives of their own.
I can see MIL is a bit disappointed every time something doesn't fit with her vision.
When we leave at 6pm on a Sunday because we want to get ready for work, instead of staying til bedtime.
When we plan a get together with our friends for bonfire night rather than with her.
When we choose to pick up from nursery rather than having her pick up.
When I refused her offer to drive me to the hospital when I was in labour (I live 10m from the hospital - she's an hour away from us!).
Whenever anything isn't how she imagined it, she's just a little bit sad.
She'd be so much happier if she stopped visualising exactly how things would go (and we'd not feel like we were constantly batting back sometimes insane logistical suggestions).