Because, much like the piano, you can easily play chords and sing at the same time. You can't do that with a drum kit or a flute. Sorry.
After playing guitar for a while you may have found that you want to write your own songs rather than just regurgitating other peoples.
If so, then I applaud you. It's a great hobby and can even lead to you being the star of the party, if that's your heart's desire.
But I'm afraid you need more than just a guitar to write great songs.
In fact, you don't even need a guitar.
What you do need is the ability to hear melodies in your head and put lyrics to those melodies.
Mel Brooks composed The Producers by singing the melodies into a tape recorder and a musician wrote them down in musical notation. No piano, no guitar, no software.
However, since this is a guitar site, we'll assume you have a guitar in hand and are ready to write some songs. Being able to play the guitar is a great advantage but here are two warnings before you plunge into your favourite chord progressions and start howling at the moon.
- Don't start writing by playing chords. Whaaa?! I hear you cry. I know, you just spent ages learning chord progressions, riffs and scales. But be aware that these are simply tools to translate the music you compose in your head. If you start with the tool, it will lead you to sounding just like everyone else who uses that tool. And there are a lot of guitarists in the world who know the same chord progressions as you.
- Don't sit down with your guitar and attempt to write a song. Again, this sounds crazy but it makes complete sense if you think about it. It's like a writer sitting down staring at a blank Word document with nothing to say. Do you have a melody or a lyrics yet? No? Then leave the guitar alone. (Exception to the rule are professional songwriters who often would go to studio to 'work' just like a job - it's very hard to do. That's why they're professionals)
So with that, here are the essential things you need to write great songs.
- An idea. This could be a riff, a melody that you have in your head or a story that you want to write. But you need at least some kind of idea before you start crafting your song. Otherwise the pressure of having to come up with something will leave you blank, or even worse, cause you to write something mediocre.
- Genuine Feeling. Only write about things, emotions and situations you understand or can vividly imagine. For example, you don't have to be a real spaceman to write a song about space travel. What you do need is the imagination to put yourself in the shoes of the spaceman and describe his situation and feelings.
- Simplicity. Stick to one idea. Keep it simple. Listeners don't have the time or the interest in trying to translate your complicated plots and roller coaster feelings.
- The ability to translate your idea into music. This is where your guitar playing comes in. You need to sit down once you've written a riff, melody or even the whole song in your head and translate it into something 'solid' using your guitar, your knowledge of song structure and how chords work with each other.
- The ability to show and not tell. This is key to virtually every creative endeavour. Show don't tell. Let your listeners see and feel what you see but don't tell them directly. Instead of saying "love is not fair, I always seem to lose", you could say "The winner takes it all".
- Don't use too many chords. When you are ready to write your chords and marry them to the melody in your head, try to keep it simple. Most of the Beatles songs are no more than 4 or 5 chords. 99% of Chuck Berry songs have no more than 3 chords.
- A quiet place. You need somewhere where you can feel comfortable making mistakes whilst singing your melody and crafting it into your masterpiece. You'll sound awful at times as you try to cobble together the lyrics, melody and chords so you don't want your flat-mate mocking you. It will only undermine your confidence.