The majority of musical instruments can be played equally well by those who are either right handed or left handed, and switching from a right handed musician to a left handed one with the same instrument is not usually a problem. However, in the case of the guitar this isn't always true, and although some models of guitar can be handed over from a right handed person to a left handed one without any trouble or difficulty, there are many cases when a guitar is very clearly designed for someone who is specifically right handed that would prevent a left handed person from being able to play it at all, or at least, not effectively.
Guitars can often be constructed specifically for a left handed or a right handed person, and there is a marked difference between the role of the dominant hand and the secondary one. So in the case of a right handed person, the dominant hand would be their right one, with their left hand being considered the secondary hand. It is the role of the dominant hand to pluck or strum the strings of the guitar, with the secondary hand being used to hold the strings against the fret board or neck of the guitar to construct the chords to be played. The reason for the hands being used in this way round might seem odd at first, since, for a right handed person, it would appear that the left hand is doing more work through constructing a complex sequence of chords.
Often, for a beginner, one of the greatest challenges lies simply in being able to train the secondary hand to physically bend, twist and stretch to the chords, because our secondary hands are not as dextrous or flexible as our dominant ones, and even though in normal everyday life this is never really noticeable (unless you try to write with your secondary hand), when playing a guitar it becomes very noticeable. However, the reason for playing a guitar this way round is because, once the chords are learnt and achievable, most of the expression of the music is generated through the strumming of the strings by the dominant hand, and the dynamics, tone and colour of the music comes almost entirely from this hand.
Our dominant hands are better able to create the very subtle distinctions in playing style which produce the expression of music, and overall quality, whereas our secondary hands are far more clumsy, and whilst we have the ability to instruct them the basic chords, teaching them the subtly of tone and dynamics would be an nearly impossible task. The same method of playing is true for those who play the violin, with the left hand, or secondary hand, being used to hold the neck of the instrument, and play the chords, with the dominant hand holding the bow which is responsible for giving the instrument the voice, tone and colour of the music.
There are some people who do not believe that the option of playing with either left or right hand should be available, and that everybody should play the instruments in the same way, with left handed people holding a guitar or a violin in the same way as a right handed person, but these people are generally a minority. No information is available as to whether these people are right or left handed however! It is unlikely that a standardisation of guitars and violins is prone to occur, since there's already a wealth of styles, designs and many custom options available to players, and many people develop their own style of music and performance through the natural way in which they hold the instrument.
Certainly with famous left handed guitarists including the likes of Jimi Hendrix, Paul McCartney and Kurt Cobain, it is unlikely that left handed guitarists will be overlooked.