My advice to Assume Love applies only in a marriage or another relationship in which both partners have made a commitment to the longevity of the relationship. Until then, it might be wise to Assume the Worst.
When you assume love, you deliberately seek out evidence of your mate's love for you. Any actions with an ambiguous cause you can safely attribute to love because you have a partner committed to sustaining the relationship, even if that commitment sometimes wavers. You can view any hurtful behavior in the context of years of loving behavior and an intimate knowledge of who you're dealing with. To do so, you must repeatedly draw on that memory of when you knew without a doubt you had found a wonderful person.
None of these will help you determine whether you've found a wonderful person. In fact, they may well obscure the truth about people you date. You will fare much better if you repeatedly assume a deceitful, untrustworthy person who seeks to manipulate and exploit you, and try to explain the actions you've observed from that what-if picture. The more often this fails, the more likely you've got the sort of person to whom you can commit yourself for the rest of your life.
Picture two overlapping circles: on the left, all of the things an unloving person of little character might do; on the right, the things a loving person of good character might do. A lot falls into the overlapping area. After you marry, the danger lies in failing to see the love in the actions that fall into that overlap. While choosing a mate or even spending time with intimate strangers, the danger lies in inventing a story that you are loved and in good hands based only on actions in the overlap.
If you don't yet know the character of the person you're with, or if you haven't yet received any pledge of future love, it's best to assume a temper tantrum reflects an inability to act in accordance with intention, a promise of future generosity without any now marks a manipulative spirit, lies about little things foretell lies about bigger things, and so on.
Yes, a loving husband may cheat on his wife, and if she assumes love she may understand and eventually forgive him because of the man she once knew. But if he's cheating on her with you, he's a man out of integrity with no history of anything but that with you. There's no "man of great character" image to draw on, no promise of one in the future.
While someone courts you as a possible mate, you'll see the best behavior they can currently manage. You don't need to give them the benefit of the doubt. And if you do, you risk selecting someone with whom you will find it much harder to assume love when it's time to do so.