I think I heard that being "in love" can only last for 6 months, and that includes when the attraction levels are at their highest. Eventually, the oxytocin wears off, just like when you take a drug and it wears off. I've found that this is why it is SO important to have deeper compatibility, and tho Ive not some deeply religious person, I've constantly found those with deeper compatibility, like being both devoutly religious, last much longer and are much more happy.
In the age of hyper choice, me me me and ultra secularism, people are more miserable than ever.