The people you hang out with, make you. This is true for friends, significant others, and coworkers. I've noticed that when people meet for the first time (assuming they don't hate each other :D ) they try bits of each other's personality on. See if it fits, ya know? It's an interesting process.
The most obvious place to look is words that someone starts using shortly after meeting someone (and spending time around them). There are subtler things like taking on someone's nervous twitches or reactions, or changing speech patterns slightly to match up better. Music, too. If you look closely enough, or know the people well enough, you can even see bits of their worldviews blending. That's the coolest thing to see (especially if it's an improvement, ha).
And remember, everyone is a multifaceted person, and you'll only get to see a few of those facets. Others will pick up on shifts you didn't notice, just as you'll see shifts others don't notice.