Why do women seem very interested in you when you're in a relationship but barely notice you when you're single?

Everytime I'm with a girl outside I get a lot of attention from other girls, they be staring and checking me out non stop... but when I'm alone or single, yes there are some women who are interested, but it's no where near the amount of attention I get when I'm with a girl.
Why do women seem very interested in you when you're in a relationship but barely notice you when you're single?
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