Why do guys joke about being in "love" or "marrying you"?

When guys joke like this with you like repetitively do they mean it or are they just flirting? My male roommate and guys friends say stuff like this like, "why don't you marry me already?", "wanna just get married?", or like after joking around they say they love me, "You know I love you, right?"

But I don't ever take it seriously because it's always in like a jokey manner and they say it like all the time so it's lost any kind of "real" potency.

Anyway, just wondering why guys would do that.
Why do guys joke about being in "love" or "marrying you"?
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