The love that is pure, real and selfless might come with a lot of ups and downs because it prepares you for becoming your better version. It helps you & your partner in growth in all three aspects - mentally, physically and spiritually. It stays with you till the end because the connection you make is pure and sacred. All the doubts and conflicts will dissolve into the ocean of love.
On the other hand, the love (what you think is love but actually it's just a shadow of love) that destroys you completely. You may feel pleasure and all good in the beginning but you know it's temporary because it has a self-motive, a plant of artificial love which you've cultivated from the seeds of deception & wrong intentions. Bitter seeds won't give you sweet fruits. It can not face the light of truth. It never meant to be in your life because it comes to your life only to teach you an important lesson, to show you where you're weak.