It's not literally true, but it's a great way of thinking about who you surround yourself with and the subtle ways they influence you.
I know I tend to pick up aspects of the people I spend a lot of time with. When we used to visit our family in Queens I'd come back talking like someone from Queens. When I had a boss who would badmouth the company a lot, I found myself also having a negative opinion of the company. Cynical friends made me cynical.
My boss now is a ray of sunshine. Just a delightful and decent human being, and I am a better person for being around him. I can only hope some of my positive characteristics rub off on others.
I'm a widower, and one of the ways that loss has changed me is that I am much more cognizant of, and grateful for, the other familial and platonic relationships in my life. I wouldn't be getting out of bed every day if not for my kids, family, friends, colleagues, and neighbors. I am surrounded by love and support.
I certainly have bad days, but I would not be where I am without the relationships in my life, and I absolutely believe in the power of those positive people to have a positive impact on me.