I believe it was Jim Rohn that said “You are the average of the five people you spend the most time with”, i.e. that your thoughts and personality (and consequently actions) will be shaped by the people around you. I don’t know if his statement is exactly right, but I’m willing to wager that it’s roughly correct. Which naturally prompts the curious-minded to ask: Why? And so what?
I can see several mechanisms to account for ‘why’. One is that, in my opinion, humans fundamentally want to agree. Or rather that our brains always do what they can to remove us from conflict, one way or another. At the same time, any two people are inevitably going to disagree on certain points, which represents a (mild) conflict that our brains seek to resolve. Many disagreements will be resolved through a bit of discussion, but a few core beliefs might well resist such resolution. What then? Since we’re discussing the people we spend the most time with I’m going to assume that avoiding exposure to the disagreement is not a likely solution. Eliminating further the option of beating our friend to death with a hominid thighbone, it seems to me that our brains would be incentivized to escape the friction by shifting beliefs and assumptions until they are sufficiently aligned with the other person’s. Of course you could argue that spending more time in disagreement simply allows one to rationally appreciate the other person’s point of view, and I fully agree that that’s part of it. But the subconscious ‘conflict avoidance’ behavior shows up in so many other scenarios (and is so evolutionarily obvious) that I think it would be foolish not to ascribe it a part as well.
Another possible mechanism stems from the idea that conversations are an important way for us to process our thoughts. The degree to which this is true for each of us varies1, but I don’t think anyone would deny that in addition to (and sometimes instead of) exchanging information, conversations allow us to permute and examine information we already have. It stands to reason that if we use conversation to process our thoughts (and tend to avoid solitary contemplation when possible), then what we talk about is what we think about. And thus whatever we have the most opportunity to talk about is going to constitute a large part of our thinking.
I don’t know how much this affects our deliberate thoughts, but it definitely plays in when we’re done with work, tired but without a specific agenda. We join our friends or housemates in whatever they happen to be discussing, and this discussion will stick in our heads as we go to bed. The next time we meet these friends, it feels natural to refer back to previous subjects. Before we know it we’ve started caring more about the issues of whatever group we spend time in, and have forgotten some of the things we used to care about. Our values shift a bit, and over time what we know about shifts as well. Conversely, this means that you are less able to develop your understanding of anything that your community is unable or unwilling to discuss (in casual conversation, mind you).
Which brings us to the ‘so what’. Namely: If the people we spend the most time with shape us to such a degree, should we perhaps make more conscious choices about who we spend time with? Only I don’t think this should take the form of ‘politely edit your friends‘, which is how I’ve mainly seen Jim Rohn’s quote treated on the blogosphere. For one thing, I think this advice is likely to fall flat once you close your browser and try to implement. And as far as I can tell organically developing friendships fare a lot better than deliberately created ones. Not to mention that focusing on your present situation seems quite likely to address symptoms rather than root causes of whatever you feel is wrong with your life. My advice would rather be: don’t edit your friends, but set yourself up to make the friends you need2.
As an example: When I was a teenager I decided that I wanted to go to MIT, based simply on the idea that it was the best university in the world for sciency stuff. When it came time to apply I gave the matter more serious thought: I concluded that it was not worth paying a lot of money for the higher quality education MIT would provide, since I could indubitably improve my education free of charge by increasing my own effort. Although this reasoning still rings true, I now feel that it completely misses the point of going to MIT: the added value is not (primarily) what the professors are able to teach you, but who the other applicants are. Anybody that gets into MIT is smart, and ambitious enough to devote time to jumping through the hoops the application involves. These are the people you’ll be spending 4+ years of intellectual maturation and youthful initiative with, and some will likely become lifelong friends. So if the personality osmosis described above is real, pre-selecting for brains and ambition might well make a big difference in what kind of person you end up being. Is that worth a hundred thousand dollars? Maybe.
 I imagine that those who are close to me might say that I’m incapable of developing ideas without speaking aloud, and I wouldn’t argue with them. Although perhaps this blog will make their burden somewhat lighter.
 Another way to influence your influences would be by spending a lot of time reading; reading a book (and thinking about it) is not entirely unlike a conversation with the author, and you have some seriously smart partners to choose from here. The catch is that you need to spend serious amounts of time reading, which lacks certain rewards for a social animal, and that you need to work a bit harder at establishing a real conversation.