This usually happens because unbelievably large numbers of
songs are written in the
Top 10 Most Overused Chord Progressions of All Time. What more or less happens is, even though while you may happen to come up with some of the chords on your own, based on the way that the mind works, as soon as you come up with a
sound you've heard before, you
subconsciously start thinking of similar songs, and the
melody lines that have been fed into your skull a billion times over the
radio start spilling out, sometimes before you realize that that's what they are.
Another common ailment for music writers is writing songs that all sound the same, which happens when you limit yourself to a certain kind of sound, or certain kinds of chord progressions, or simply imitate the sound of a limited musical genre.
The remedy to these kinds of problems is creativity, which (more or less) what seperates musicians who stay around for a while from one-hit wonders, who are popular briefly because they fit in with the radio sound du jour. Don't just sit around with your guitar and play chords until something sounds good; even if this works pretty well sometimes, it seriously pays off to learn a little music theory, test out how different scales and harmonies work when wound together, and combine improvisation with lines that you write out on paper in advance.