Nearly two decades ago, Jack Nicholson handed generations of men a line that actually worked.

“You make me want to be a better man,” he told Helen Hunt in As Good As It Gets.

It’s impossible not to respond to a line like that, if spoken honestly. Is there any greater compliment? You inspire him to be a better person, to be worthy of your love. Even if things don’t work out, you’ll always know you changed his life. You changed him.

But why does that line work so well?

What does science have to say about the relationship between love and self-improvement?

Quite a lot, it turns out.

Relationships help us grow as people. We learn to communicate and compromise. We learn to give and receive love. We negotiate the delicate balance between self-care and service.

And we also become bigger people, through a process called self-expansion.[1]

In everyday life, we tend to think that who we are stops at the boundary of the body. This is me, and everything else is not me.

If we look more closely, we realize that’s not true. Psychologically, we are defined by our relationships. We embrace those we love as part of who we are. We wouldn’t be who we are today without our friends and family, who’ve shaped what we like and how we think.

The same goes for our romantic relationships.

Think about the last time you fell in love. Chances are, you felt as if you were merging into your beloved as if you were no longer two separate people but rather one. Falling in love helps dissolve the boundary between self and other, thereby expanding our sense of who we are.

That feeling of self-expansion is so important that it can make or break relationships.

Dr. Arthur Aron and Dr. Gary W. Lewandowski, Jr., studied the self-expansion model of close relationships. Their research showed that partners who grow personally from being in a relationship feel greater satisfaction and commitment.[2]

No one actually sets out to fall in love for the purpose of personal growth. But it’s a pretty good reason.

The best relationships introduce you to new ideas, resources, and experiences. Your partner shows you a whole new world you never imagined before. You learn from him, and he learns from you. Even if the relationship ends, you’re not the same person you were before. Everything he has taught you becomes part of you.

Our personalities, preferences and perspectives change as a result of our romantic relationships.

For example, if you’ve always admired funny people and you date a man who’s got a great sense of humor, you may pick up some of his talent with comedy.

If your partner is ambitious, but ambition was never high on your list of priorities, you may find yourself picking up some of his enthusiasm for goal-setting.

The longer you’re together, the more similar you will become, in a thousand subtle ways.

There lies the promise…

And the problem.

If he is someone who expands your sense of what is possible—if he makes you want to be a better woman—then you’re fine.

But you may find that his world is much smaller than yours. He’s eager to learn from you, but he doesn’t have much to teach. You’d like him to become more like you, but you don’t want to become more like him.

It pays to think twice about the long-term potential of a relationship that doesn’t help you grow.

Aron and Lewandowski urge us to ask a few questions about our relationships.[3]

• Does your partner introduce you to new experiences?
• Does he broaden your horizons and give you more perspective?
• Does he teach you new things and encourage you to try things you’ve never tried before?
• Have you discovered new things about yourself as a result of being with him?

If you answered yes to most of the above, then you’re on the right track.

Similarly, ask yourself if you help him expand his sense of self. Do you introduce him to new experiences? Do you teach him new things? Do you help broaden his horizons?

Don’t hesitate to share parts of yourself that may be unfamiliar to him. If he’s never tried your favorite foods or hobbies, make a point of introducing him. Your differences are the spice that keeps your relationship exciting.

And the best way you can keep your relationship growing is to never stop growing yourself.

Jack Nicholson was right. Love really does help us become bigger, better, braver people.

[1] http://www.oxfordhandbooks.com/view/10.1093/oxfordhb/9780195398694.001.0001/oxfordhb-9780195398694-e-005

[2] http://www.nytimes.com/2011/01/02/weekinreview/02parkerpope.html

[3] http://well.blogs.nytimes.com/2010/12/31/the-sustainable-marriage-quiz/