Any reason is valid. It's your own decision. Mine are like this:
Drums, bass and guitar - I chose those because I grew up with them, therefore I had the background and practice. Made sense to buy what I already knew I loved playing.
Wind instruments - I hate brass and reed instruments because of the way they make your lips feel. I would never try to get into those. It would just be torture.
Piano - I would love to pick this up, but I don't want to drop thousands on a grand, and under no circumstance do I think those sh*tty Casio knockoffs are sufficient and not disgusting. So I probably won't ever get into piano.
Strings - I would love to learn the cello, simply because I love the sound. Someday I'll get one. Maybe a viola too. Definitely not a violin. Too small and screechy for me.
As for world instruments, not really interested. Doesn't apply to my stylistic preferences.
Anyway, as you can see, it just depends. For me it's partially my background, tastes and dislikes. Whatever the case, it's all pretty relevant to me at least.
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