S.T.
To me it's the nature/nurture element. We are all born with a certain personality that's hard-coded and then we have life experiences that affect us. When you can get a better picture of your grandparents - their personality & style, and then your mom's childhood and life experiences it helps you to see the full picture of who your mom is.
My mom was born to European immigrants in 1930. She had a serioius virus that affected her eyes (that she got from her dad due to carelessness in his life before he was married - long story) and spent much of her childhood vision impaired, traveling to doctors and spending time in hospitals. He childhood was in the Depression, her teen years were during WW2, she married an alcoholic, etc.... All of that made her the stubborn, spoiled, headstrong woman she is. It also enabled her to survive ANYTHING and to be an amazing example of resiliance and fortitude.
My MIL's mother was nastry & crazy (so I'm told) and her dad was a drinker who would do anythign to keep the peace - but was a happy drunk. My MIL suffered from depression, her husband became a cop and she spent month in a psych hospital after she had twins (my husband and his brother).
Knowing what went on in their lives makes it soooo much easier for me to understand them and identify what's "personal" and what's not. Most of it isn't personal - it's just people responding to triggers from their own life.
Good luck - it's often most difficult to deal with those closest to you. My mom is now dying of Ovarian cancer at age 82 and I now look back to those times that she infuriated me and I responded badly, etc. I am so much more patient now knowing that she's in her final days and I'll live with every word I say for the rest of my life... If only we lived this way about all the people we love!