What is the psychology behind the “opposites attract” theory?

Actually, the reality is different. Enough studies confirm that we are attracted by people who share similarities with us (more details here: Relationships: opposites do not attract, scientists prove).

Just remember all the times you met someone new. The conversation was just “ok,” until you found out that he/she shares the same musical taste, political view, or whatever. Then you feel excited and say, “Do you like ‘this’ too? I love it!” And then both of you are excited and start to talk more about the subject.

The reason why people say opposites attract is that we don’t notice when they agree, because that’s what we expect, but we pay attention to every minor disagreement they have in opinions, preferences, and beliefs, and so we tend to believe that couples are so different, when they in fact are more likely than we’re able to realize.

A little example: if you see a couple formed by a Christian and a Muslim, we could say “opposites attract” because they are from different religions. But then, we would fail to realize that they both share the same belief in being tolerant and embracing diversity. We’ll NEVER see a couple formed by a radical Christian and an extremist Muslim.

There are a lot of differences that are not deal brakers. As in the previous example, religion is not a deal braker, but religious extremism is. But we always will pay attention to those differences (that are not deal brakers) and ignore the similarities (that are the real reason why the couple is attracted with each other). 🙂

Extracted from my answer to a question asked on Quora.