What causes our whole lives to change when we meet the one we will end up being with?

It's impossible to be in a long-term relationship and not have your life change. For starters you both will meet each other's friends, families, and co-workers. You share, compromise, and support one another in countless ways. Had you not gotten involved with your mate you would not have to "consider" how your actions effect anyone other than yourself.

A relationship changes your (mindset) from "You & Me" to "Us & We" regardless of who you end up with. Our focus becomes, What is best for the relationship/marriage?