Whenever i want to learn a new song, despite having a fairly wide music taste; from Steve Vai to MJ and from Orianthi to Megadeth, i only ever seem to actually focus on songs from one band: Metallica

I do pick other songs sometimes but i never really stick at them as much as i do with Metallica songs, is this a bad thing for my guitar technique and prowess?
I started off that way with Maiden back in the eighties.

You'll grow out of it.
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If you're only going to learn to play songs by one band on guitar, you could do a lot worse than Metallica.
that's the way a lot of people are, I was like that too, only learning Tallica songs. Sooner or later you'll start to want to learn other songs, don't get worked up about it.
Not at all. Learning their entire library should keep you busy for awhile
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I mostly just write and play my own stuff. I learn riffs here and there when one catches my ear and sounds fun to play, but I usually don't bother learning anyone else's songs unless my band intends to cover them. I learned some songs back in the first year or two I was playing, though. Mostly Tool, Blink-182 and, surprise surprise, Metallica. At one point I could play along with the entire Master of Puppets album, minus the solos. Then I didn't play any of it for like 5 years, and now the main couple riffs from MoP is all I remember.

But yeah, learning songs is great practice. I just kind of got out of it once I got a pretty good grasp on the instrument, and the music I wanted to make myself.