Mother of sons

Why is it that mothers of sons tend to have a harder time accepting their sons partners. My mother has both sons and daughters. With her daughters husbands she was so welcoming and accepting and it was obvious with my brothers wives that she was a little bit more standoffish and although she is very nice to them it is more of a false relationship rather than the ones she has with her son in laws which seems very natural and relaxed. I tend to feel the same about my own MIL..it’s a very false polite relationship..not the relaxed one that my husband has with my family.