Michael's Withdraw from Confrontaion
Something I've noticed is Michael is not confrontational when it comes to conflict in his family. From distancing himself from his father, to Kay and avoiding her after the hit on thier bedroom, to not wanting to hear about his son's problems, and isolating himself from Fredo and everyone else at the funeral by presiding in the Boat House. And at the end we see Michael staying away from his family during the birthday party, not wanting to face them, or thier differing views and lifestyle. Then suddenly announcing his enlistment and then staying behind. Alone.
This also comes out in how he deals with enemies by moving silently in the background until the time is right. It's as if Family Strife has become another enemy to Michael. Avoidance then sudden confrontaion.