Actually, I had just read about that recently. Due to the way we evolved, our biological nature, ect. Love is a human creation as far as we know, no animals exhibit it, no animals show any form of love to each other, they find a mate and move on if they die, within mere hours. Sometimes even minutes.
So in a sense, we humans are the "weird" ones for wanting long-term relationships.
Still, tell that to woman, haah.
You have several animals who mate for life, for example swans. Love is an emotion, an emotion is a chemical reaction in your brain. A certain chemical reaction is produced in the brain when you see/hear/feel/taste/smell that special someone, and makes you feel in love. Love also exists as an chemical reaction in the animal kingdom. Remember that we humans are mammals, and all mammals share the same basic design. If we have love, other animals have love. We have jealousy, other animals have it too.
Baby elephants have died, the elephants lingered around the dead baby, trying to revive it. After a few days, each elephant walked up to the baby, touched it, sniffed it and moved away to go on their way. They were saying goodbye. To say Animals do not show any form of love is just ignorant. I would like to know were you read what you supposedly read.
Humans are weird that we force each other in 80 year relationships, when it doesn't come natural to us. Sure for some it works, but it simply isn't in our nature. It isn't how love works. Marriage was invented by religion to insure many children are born, who are raised with the religious view of parents.
Love works for several years, but your system gets used by the chemical released in your brain(Just like with drugs, you need more each time to get a similar effect). You become resistant to love. This period was about 8 years I believe, in which the mother will have pooped out a baby and the kid is getting close to being selfsufficient (Not based on current standards). the dad goes off to get another chick pregnant to spread out his DNA as much as possible.. Just like certain/most mammals do. The reason marriages do work is that love is replaced by something different. A feeling of being happy together. The transit between love and this mindset(well it's really just another chemical reaction too) tends to be the most feeble point in a relationship.