Meaning of "Everybody loves a lover" idiom [closed]

While doing homework for "Introduction to Mathematical Thinking" course on Coursera I came across the phrase "Everybody loves a lover". At first I thought that the meaning of it is straight-forward, but then the task description said it was an idiomatic expression. The exact problem description was:

Which one of the following means "Everybody loves a lover", where L(x,y) means (person) x loves (person) y and a lover is defined to be someone in a mutual loving relationship? (If English is not your native language, you might want to discuss this sentence with a native English speaker before you answer. It's an idiomatic expression.)

I couldn't find the meaning of it on the Internet, so I'm asking here: what is the exact meaning of this expression?

Solution 1:

The expression itself seems to mean that it’s easier to be friends with a friendly person, and it’s easier to be in love with one who is loving.

However, the word lovers also is used to refer to people in a sexual relationship. So, while I’m loving toward my children, I’m not my the lover of my children – not in that sense of the word.

However, the word lover can also mean someone who enjoys something very much. I can be a lover of homemade ice cream, or classical music.

I think the caution in your book is referring to this dual sense of the word – that is, lover doesn't always merely mean someone in a mutual loving relationship. Sometimes the word lover can be used innocuously (like, I’m a lover of history), and sometimes scandalously (as in, I’m the lover of my wife's cousin).

From Macmillan:

lover
1) someone who is in a loving or sexual relationship with another person
a. the sexual partner of someone who is married to another person
b. used for talking about someone's sexual performance

2) someone who likes or enjoys something very much

From Collins:

lover
1) a person, now esp. a man, who has an extramarital or premarital sexual relationship with another person
2) often plural either of the two people involved in a love affair
3) someone who loves a specified person or thing ⇒ a lover of music
(in combination) ⇒ a music-lover, a cat-lover

I think the book simply doesn't want someone to use the word in an embarrassing way.

Solution 2:

It means that those in love are popular because, suppsoedly, they spread their happiness all around them. It's not really an idiom, because the meaning can be deduced from knowing the individual words. (What's it doing in a mathematics course, by the way?)