Love Songs

How do bands like The Beatles come up with so many great love songs?
I mean seriously. They have come up with so many songs, almost all about love and almost all ridiculously popular. How do people come up with them? I know that some of the songs are based on experiences or real life happenings but they can't all be. I am pretty sure that between the four of them, The Beatles couldn't have fallen in love with the amount of women that are in their songs. There are billions (okay maybe not billions). Lovely Rita, Eleanor Rigby, Molly (Ob-la-di), Maggie Mae, McGill and she called herself Lil, but everyone knew her as Nancy (Rocky Racoon), Long Tall Sally, Dizzy Miss Lizzy, Michelle, Lucy in the sky with diamonds, Lady Madonna, Joan (Maxwell's Silver Hammer), Anna, Polythene Pam, Sexy Sadie, Prudence, Julia and Martha (my dear). And there is probably a million (okay maybe not millions) more hidden within lyrics of their songs (or that I just haven't found) but still, that's a lot of women. But could they really fall in love with all of them? John Lennon I know cheated on his first wife and I think I have read that Norwegian Wood is a confession of that  but thats only one song.
My sister went to the National Graduate Show in Perth which show cases 35 artists recently graduated from arts degrees from all over Australia. One of the graduates there was a girl who wrote songs as her art work. She would get people to answer a list of questions about a certain experience and she would go and make a song about it. Some of the questions involved were "what did you feel?" or "what colours would you relate to this experience?". I think its a really interesting concept. And such a good idea. And some of the songs are so funny. One is called 'The Bronken Penis Song'. Cracks me up every time but some are really sad. If you haven't had the experiences, write about someone else's. But I have never heard of The Beatles doing this.
So how do they/did they do it?
Bands that are less popular or that are only one hit wonders then I can understand that, they are only writing one song but The Beatles have hundreds of songs and that is no exaggeration. Well maybe I don't need to understand how but just appreciate the result, which I do, probably the greatest band that ever lived, just saying.