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The simplest explanation I have heard is that there are an infinite number of possible orbits that a planet of given mass and speed can adopt, all of which are ellipses. Only 1 of those orbits as actually circular ( the circle being a special case of a ellipse with both focii being in the same place).
So while a perfectly circular orbit is possible, it is highly unlikely.
In a simply 2 body system, it is more likely that the orbit will be highly elongated, again simply because there are far more possible orbits that are highly elongated than ones that are nearly circular. And this is what we see with double stars where elongated orbits are the norm.
In a system with multiple planets the interactions of the various bodies combine to make the plantary orbits more circular over time, hence the planets of our system have only small elongations. For example the Earth eccentricity is very small. The planet with the largest eccentricity is Mercury, which fits as it would be the least influenced by the interactions of the other planets relative to the Sun.
Hope this helps in a non technical way to explain it as it was explained to me.
Cheers
Malcolm
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