Who we are and who we become depends, in part, on whom we love.

The idea here is that identity is not something we build alone and then carry into our relationships. It is shaped, quietly and continually, by the people we attach ourselves to. The word "love" matters: we take on the moods, habits, and expectations of those we are close to, often without noticing, and they become part of how we see ourselves.

The quote is usually credited to Thomas Lewis, a psychiatrist who co-wrote a book on the science of emotional bonds. Because the name is common and I can't confirm the exact source, the attribution deserves a little caution. The thought fits modern attachment research, which holds that human nervous systems regulate and develop each other, and that early and later bonds leave lasting marks on temperament. It also echoes older views, from Aristotle's ideas on friendship to the common saying that we become like the company we keep.

In practice, someone might notice that after time with a certain friend they feel smaller and more cynical, while a different friend leaves them curious and more generous. Seeing that, they could spend more evenings with the second friend, and bring that friend's warmth into their other relationships.