Love means infinite forgiveness.
Although I understand where that statement likely originates, I don't believe it is ever true. It is an ideal we lift up because we've heard it and we want to believe in a love that is absolute. Christianity uses "unconditional love" as do many pet lovers, both of whom are wrong. Not even God loves unconditionally.
The list below contains some extreme reasons why people stop loving, and some mundane. There are those who argue that if you stopped loving, then it was not love in the first place. I find that a bit too convenient, as it makes the situation fit the construct instead of the reverse.
1. You loved your cousin until you learned that he has been killing cats for sport since he was eight. You had no idea, but now you do.
2. The father of your children has been a good provider and a helping partner, but you learn that for the entire marriage, almost, that he had a second wife and kids and lied to you more times than you can count about where he was and what he was doing. Trust is shattered.
3. Your estranged daughter became a meth addict, and in a delirium, drowned your three grandchildren, all under the age of seven, because she was at her wits' end and knew she was going to prison or die and didn't want her ex getting the kids.
4. Your husband gets fat and you've never been able to accept fat people. You keep saying the words that you love him, but you honestly don't. Deep down, you're angry that he doesn't care any more and the outer beauty that was such a big part of your love life is now gone. You feel guilty for not being able to get through the superficial, so you stay angry all the time, and hate it.
5. Your husband fell out of love with you but continued keeping up appearances and you missed the signs after being together for over a decade. He left and cleaned out your joint savings account and fled to a country with no extradition treaty. You don't know if he has a partner with him or is just out there having a mid-life crisis, but blowing through your hard-earned money.
6. Your loved one develops pre-senile dementia at 50. He's otherwise hale and hearty, but it's obvious he's losing his mind quickly, and his paranoia makes him afraid of you, angry, and sometimes violent. You want to stand by him until the end, but the man you loved is simply not there any more, and as he goes into institutional care, you're not sure you can keep up the front of pretending to still love him when he's a shell of who you loved.
All of them happen. Some rarely. But they happen. And the exception proves the rule. Many people SAY they still love because they've been told they should. I'm not sure how real it is when anger and fear outweigh the memory of love.