Nowadays people have forgotten what love really is. it isn't only passion and good feelings. Love is also hate and anger, sad and sorrow. Forgiveness and sacrafice made those feelings love and if love somebody you sacrafise and forgive even it hurts. That's love.
As a my point of view it is not necessary to love somebody throughout your life but if your love is true then it will happen automatically.
Trust is must for maintaining any relationship till the end. Another thing which need in love is feelings for your partner which make you love your partner thought your life.