Jim Rohn (the notorius self-help guru, yeah, I know) said that you are the average of the five people you spend the most time with, so you should choose carefully who those people are. This makes sense up to a point: you will adapt your friends' habits and ideologies if you hang out with them and listen to them a lot. But see what I did there? I made one crucial and erroneous deduction: that you spend a lot of time with the people you spend the most time with.
There's a fundamental difference! What if you're a loner whose set of five people include the mailman (1 minute a week), the cashier in the grocery store (5 minutes a week), their father (the occasional phone call every couple of months), their neighbor (casual "hellos" three to four times a week), and their cat? (I know a cat isn't a person, I'm just trying to make a point.) The loner knows hardly anything about the first two, perhaps only their profession. And if you don't spend time face to face with your dad, how can his lifestyle choices and ideologies rub on you? All I know about my neighbors are their surnames and addresses. So, basically, the loner is the average of the cat.
But sure, I'm only talking with my own personal perspective in mind, and I wouldn't want to be the average of the five people I spend the most time with (apart from my spouse, I don't spend a lot of time with anyone). Generally speaking, though, people are social animals who have friends and acquaintances and a lot of family with whom they are close. And if that's the case with you, then I agree with Rohn and suggest you analyze whose attitudes and attributes and lifestyles you want to catch on to you. Because they will catch. You don't want to lose those people from your life (at least subconsciously, in terms of social Darwinism), so to ensure that they like you, your subconscious will make you talk, dress, eat, behave, and exercise like them. If they smoke and eat unhealthy, you'll start to smoke and eat unhealthy. If they are climbing the corporate ladder, you'll start doing it, too. If they complain about external circumstances and other people, you'll start complaining as well.
If you want to have progress in your life, choose friends whose attributes you want for yourself. But once you've got them, remember to start looking for bigger game.