I think it depends on personality types more so than instrumentalists, however certain personality types are attracted to 'lead guitar' and 'lead vocals' over other instruments.
People who call themselves 'lead guitarist' sometimes have ego problems, because that very term identifies themselves as superior through the use of the word 'lead'. The need to define some kind of superiority or the fact they call themselves 'lead' can in some cases be a warning sign of an ego problem. Same can go for 'lead singer'. It depends on the band. Never heard of a lead bassist or lead drummer
I just call myself a guitarist. I've just begun playing with a new band and so far I'm doing the 'lead' parts but I don't feel the need to define myself as 'lead guitarist'. It seems an unnecessary definition to make. The other guitarist is as important as I am in terms of delivering the whole piece of music.
Being able to play technically challenging pieces on the guitar isn't special or unique anymore, just check out youtube and you'll find thousands of guitarists better than yourself, and they're not famous either.
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I think it is all about maturity. A mature musician will realise they are just one part of the big picture, an immature or egotistical musician thinks mostly that they are the focus of the picture and that the other parts don't matter or contribute as much as they do.
Even if someone is more talented it still pays to be humble and recognise the contributions of others. I gig regularly in a samba band where gigs have 10+ drummers. Some of the percussionists are pro-drummers with either successful bands or teaching jobs, and the rest of us can only play to the level needed for the band. The better guys are fantastic in that they help teach technique to the 'non-drummer' percussionists, as at the end of the day everyone needs to play well for the whole band to sound good.
I think it would do many people good to play a short while in a samba band or other percussion group. The very nature of each instrument means that in isolation parts can sound naff or boring. However when all play together it sounds fantastic. Really hits home the 'only part of the bigger picture' thing. Have a go on this!
http://carnaval.ig.com.br/rio/carniv...135402474.html
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Guitar is only one instrument in a typical rock band, and one single musician can't carry the entire band. In my experience most people are listening to the vocalist, and only listen to other instruments as a complimentary part of the package.
It tends to be only musicians or people with a special interest in an instrument who really pay attention to specific instrumentalists such as the 'lead' guitarist. Otherwise, to an extent we're all just backing musicians for the singer to the average listener. In this scenario an egotistical 'lead guitarist' is wasting their breath and the rest of the band's time with ego because not many people really care.
I'm not saying let the singer have an ego problem, but I'd argue guitarists having an ego problem is the worse of the two options.