Delve into the culture of love songs by listening to a wide range of different artists and genres. Figure out what you like about the songs' lyrics and what you dislike. Perhaps you enjoy the simplicity of one song but dislike the sentimentality of another. Notice the rhyme and rhythm of the lyrics and how that balances with the music. This should inspire you to try to perfect your lyrics by listening to those most renowned for their love songs.
Talk to other people about love, asking them about any experiences they've had, good or bad, with relationships. Remember that love songs do not have to stem from exact personal experiences; just from universal feelings human beings share concerning love. That being said, also remember some times in your life where you loved and how that affected you.
Watch romantic films to get inspired. After watching a really poignant movie, you should feel more moved to write in better detail the essence of love and how it affects you and those around you. Try an experiment where you create a song detailing the story in a romantic movie. Afterward, go over the lyrics you wrote and see what you liked and what you disliked.
Decide on your strengths and weaknesses as a songwriter. If you feel confident in a more direct storytelling approach, perhaps you should stick to it when writing lyrics as opposed to a more metaphorical style. Know that every writer is different in his approach to writing love songs, so you have to figure out your strong points and continue to perfect them until you create a love song you can sing to move those who hear it.