Loving relationships alter the brain the most significantly.
Just consider how much learning happens when you choose a mate. Along with thrilling dependency comes glimpsing the world through another’s eyes; forsaking some habits and adopting others (good or bad); tasting new ideas, rituals, foods or landscapes; a slew of added friends and family; a tapestry of physical intimacy and affection; and many other catalysts, including a tornadic blast of attraction and attachment hormones — all of which revamp the brain.
When two people become a couple, the brain extends its idea of self to include the other; instead of the slender pronoun “I,” a plural self emerges who can borrow some of the other’s assets and strengths. The brain knows who we are. The immune system knows who we’re not, and it stores pieces of invaders as memory aids. Through lovemaking, or when we pass along a flu or a cold sore, we trade bits of identity with loved ones, and in time we become a sort of chimera. We don’t just get under a mate’s skin, we absorb him or her.