In the great war of light vs dark everyone is called to choose a side, from the lowliest beggar to the mightiest wizard. Truely neutral individuals are rare, almost everyone has some leaning one way or the other. For heroes that is doubly true and few powerful heroes are able to maintain a neutral stance on good and evil in the face of the forces attempting to recruit them. Most of those who believe they have maintained a balance have in fact been snared up in some plot or other and are fighting the fight without realising it. Worse, since the good powers are more interested in conversion than dominating, these plots are usually of evil and hence neutral characters tend to slip into darkness.
A PC chooses his alignment at the beginning of the game, but it is a fluid thing and good / evil represents more than their innermost feelings. It represents which side they have chosen. Changing alignment means turning ones back on ones side and not all powers are happy to allow that choice to be made freely.
In game terms, a player chooses the alignment for his character. A neutral character will find themselves manipulated between the two forces as they are encouraged to undertake tasks which will bring them closer to each. They can at any time choose to become evil, however to gain a good alignment they have to behave in an appropriate way and earn it. A character who is good can at any time renounce their alignment and slip as far as they choose. Equally, a good character who has been behaving in an evil way may find themselves slipping anyway. An evil character can atone, but it is much more difficult. If they can find a good cleric to cast atonement on them (note: this cleric may not be a travelling companion) they can undertake an appropriate task and climb to neutral. From there, if they truly want forgiveness they have to work for the light to become good. Although forgiveness is easily forthcoming, they have to work at being trusted.