Uh oh! Here comes another "novel" from theFixer...
5 bux & my left nut says it's because he doesn't feel like the song reflects enough of him, & he wants/needs to put his stamp on it too. It just comes along with the drummer's territory. I would imagine that HE doesn't even know that's what's going on. My time away from the drums has enabled me to look back and recognize that, plain as day. I wouldn't be at all surprised to find out that he crashes & rides on his cymbals pretty heavily, too. Guitars, keyboards, horns... EVERYBODY but the drummer has melody, harmony, solo's, etc. They produce different notes and tones that sustain and fill sonic space... and it all happens on top of the drums. "Boom kak boom boom kak" is neither melodic nor "pretty". Everybody sees the house... nobody sees the foundation. Barring the Neil Pearts of the world... the drummer sits in the back, shuts up, & does his job. It can seem like you're there FOR the rest of the band... not WITH them. Depending on what demons he may be fighting (depression, substance abuse, pride, etc.), that can fester into a low sense of self worth, inadequacy, and more. Ever known a drummer who quit a band by surprise after being with them for a long time ("I don't know what happened man! Everything was going so well. Everybody seemed happy... he just bailed! I don't get it!"). I've even come across a couple guys who quit playing drums completely. "It just wasn't fun anymore", and "I just didn't know why I was doing it anymore", were among their responses. I'm tellin' ya... it all goes back to what I've just said. I have a couple suggestions for ways to stave that off. For one... if your drummer sez, "Hey guys... why don'tcha try this"... periodically, try it. If it sux... let him hear it suck. It's the difference between saying, "You & your idea both suck. Shut up & siddown", or saying, "Damn dude! Cool idea! Unfortunately, it won't work. Here's why". Also... as in your case... you have 3 people in the band. You & the guitar player constantly interact with each other. You discuss riffs & chords & effects & all kindsa stuff. There's the 2 of you, and then there's him. Try to pull him into the fold as much as possible (within the limits of practicality of course). Hell... Neil writes all the lyrics! I promise you, that has had a tremendous positive affect on Neil, Geddy, and RUSH. See what your boy has to offer. Though YOU know your a 3 piece band... your drummer may be feeling like it's a 2 piece band with a drummer. Yawmsayin'?