Anyone who spends enough time with me will hear me say “the person you become is determined by the books you read and the people you surround yourself with.” A great mentor told me that early in my career, and it’s been the core of my philosophy ever since.

I’ve written before about how reading a book is like spending time with a person, how I can spend a couple of hours with the great Urban Meyer (a man I don’t know but really wish I did), simply by reading his book—and how I’d be a fool not to take advantage of that opportunity. But why is spending time with certain people important to begin with? Why does the people you surround yourself with (in person or in print) matter?

Let’s look at this.

Attitudes Are Contagious

If you’re constantly hearing certain ideas, assumptions, and values, you’re likely to end up agreeing with them. That’s human nature. Even specific feelings are catching—if you’re surrounded by sad and grumpy people, it’s hard to maintain a good mood. If you’re with someone who’s cheerful and confident, some of that cheer will rub off on you. The big question is whether the people around you have attitudes you want to catch.

That doesn’t mean you have to ditch your friends who are having a hard time. It’s about balance. Are there people in your life who lift you up? Are there people who inspire you to be the best that you can be?

Habits Are Contagious, Too

If everyone around you thinks it’s normal to be sleep-deprived and over-worked, guess what? That kind of insanity is going to come to seem normal to you, too. If everyone around you is eating nothing but fried foods and talking nothing but negative gossip, you’re going to feel a lot of pressure to do the same. Look at the lives of your friends—that’s probably going to be your life, too, in a couple of years. Is that what you want?

Again, yeah, you don’t have to ditch that old friend from high school with the embarrassing political opinions and the questionable taste. You two go way back. You’re tight. That’s good. But seek people out who have the kind of life and career you want. Make friends with your heroes. They’re obviously doing something right, maybe something subtle even they can’t describe. Stick with them, and maybe you’ll catch what they’ve got.

Why Should You Have to Reinvent the Wheel?

Living well and being successful depends on doing a lot of things right. Maybe you’re particularly gifted and you can figure a lot of it out for yourself, but no matter how smart you are, you can’t figure out everything—and you shouldn’t have to. There are plenty of wise and wonderful people you can reach out to and learn from. And many of them will be happy to help you if only you ask.

Every month, I have a one-on-one mentoring session with one of the most successful CEOs in the MassMutual system. He is more than willing to share his insights with me and guide me as I work through challenges and seek out opportunities. He does not charge me a fee and doesn’t expect anything in return. I simply reached out to him in the spirit of wanting to learn and asked if he would mentor me. Not a month goes by that I don’t feel a deep sense of gratitude for his willingness to spend this time with me. Each day I seek to pay this experience forward to others. I look for ways I can make my attitude and my habits worthy of being contagious, too.

A Library of People

I wrote before that reading a good book is like hanging out with a great person. Another way of looking at that is that the people around you are like a library of wisdom and resources. But not all books are good and not all people are wise. Either way, what you read and who you’re with are going to influence you, at least to some degree; it’s like that old saying, if you put garbage in, you get garbage out. The point is, it’s your life, and you get to choose what direction you take it. Part of how you do that is by thinking carefully about which people—and which authors—you want in your life.