To be faithless in your dream means that you feel the urge for an important change in your life, not necessarily in your love life. There is something missing in your life, a void that must be filled in order to achieve balance and happiness.
A dream about you being unfaithful to your lover may also arise from the subconscious part of the self in response to monotony or instability in a love relation. If you do not feel comfortable and are not satisfied with your present relationship, you will often have dreams about cheating on the partner only to fulfill your sexual desires. But adultery in dreams is not only caused by sexual urge, it may also arise from someone's need of receiving love and affection from the partner.
Other elements of the dream are also very relevant for interpreting it. For instance, it is highly important how you feel in the dream about being unfaithful/ cheating on the partner - do you feel guilt or pure pleasure? If it is guilt, this may mean that your love relationship must be 'refreshed' and you are willing to do all that is necessary for improving both your love life and your sex life. On the other hand, feeling pleasure and no remorses may mean that you really feel the need for a change and you may try to meet new people to see if this makes you happier.
Dreams of being faithless may also be brought about by someone's desire for independence and liberty to do exactly how he wants and when he wants. This person may feel subjugated in his relationships with the others or in his current love relationship.
On the other hand, dreams of someone else being unfaithful to you are as common as those of you yourself being the one to commit adultery. If you dream that your partner cheats on you, this mainly means that you fear being left by the partner. You lack stability in the love relationship and consider that you do not receive enough affection from your lover. All these feelings arise from a low opinion on the self and low self esteem.
If the dream does not involve you or your mate, but you see other people cheating on their partners this may stand as a warning that you should not get involved in other people's business and only look out for yourself.