There isn't really a simple explanation for it, but you can get an intuitive idea from considering the elements involved.
Oxidation is the loss of electrons. So an element is more likely to be oxidised if it loses electrons more easily.
Consider metals - metals always form cations (that is, you see Na+, never Na-). Also remember Yr 11 Chemistry when you learnt about metal lattices with the loosely bound electron "sea" permeating. Final consider the periodic table, where metals are all placed on the left hand side - such that, to form a stable octet, they need to lose electrons. You can then compare this to other elements such as oxygen or fluorine which are on the right hand side of the periodic table - they need to gain only a few electrons to get the electron configuration of their nearest noble gas (ie. it would be too energetically demanding for it to get to a stable configuration by pulling away HEAPS of electrons).
Now look at the electrochemical series - placed at the top of the series are elements such as fluorine and oxygen, and closer to the bottom are elements such as sodium. The metals are down the bottom of the table, and they are generally easier to remove electrons from. Intuitively, you can recognise that the metals which you only need to remove ONE electron from to get to a stable octet are very close to the bottom - they lose their electron very easily to get to the octet!
This is a bit of a hard and fast rule and you can probably find some exceptions if you look carefully - this is because of complex chemistry that dictates the exact order of the electrochemical series. Evidence of this is the fact that the orders change when you're not under standard conditions. It does give some guidelines for thought though. At the end of the day, you need to recognise that electrons move from the from the lower E value to the higher E value. This is based on experiment (ie. complicated chemistry under specified conditions) to determine the values.
Hope I haven't confused you more!