I agree with all of y'all, that its is because Jacob(any guy or girl) felt betreyed by Bella(that persons 'girlfriend/boyfriend' or 'true love') chosing Edward(any other guy/girl that the girl/guy likes more than the first one), but also its that Jake(the guy/girl) would prefer that its that person doing it to them, rather than someone else, but all the same doesn't like that its happening. But its totally wrong to cheat on sumone, its betreaying the trust that they gave you by letting you in their heart, its better to break sumones heart ltteing them know you care for sumone else more, or don't realy care for them at all, than to rip it out from the inside cheating on them and having them find out that way.