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I'm going to be really honest. I've actually only written one love song, for my wife when we got married. When writing loves songs, you want to remember the theme of tenderness, and the emotions that are going through you. So, how do you translate that to the piano? I kind of like the key of E flat, I think it is a pretty sounding key that is good for this type of music. When you play the E flat chord, try adding a 9th note to it (you remember your intervals don't you?). The 9th note adds a little something to the chord. Maybe try going up an octave after a while. Whatever you do, just remember to focus on creating that tenderness.
Four chords that are really good for a love song are the I, IV, VI, and V chords. Trust me, with those four chords you can write a love song. In the video I play a little progression starting on the root, then moving on to the four then the six, then the five. From there I move on to the two chord and walk back up to the five. Then I take the song in sort of a power ballad direction, really digging into the root chord, the four chord, back to the root, then back to the four before ending the chorus with a five chord. Remember the five chord is great for ending musical phrases before going back to the root in the next phrase. The root is the perfect chord to end a song on. After the chorus I return to the root chord and the song gets soft again.
This is, of course, by no means an exhaustive tutorial on how to play a love song. Hopefully though, I've given you at least some direction on what chords to use and some general ideas for the structure of your song.
Maybe you just want to write a normal song. Check out the lesson on How To Write a Song.