Psychologists split love into two kinds — one that burns hot, one that just lasts

Elaine Hatfield and G. William Walster argued early romance runs on 'passionate love' — intense longing that tends to fade within a couple of years. What often replaces it, if the relationship survives, is 'companionate love': a quieter bond built on trust and attachment. Couples who mistake the fading of one for the death of the other sometimes split right as the steadier kind takes hold.

— Elaine Hatfield and G. William Walster, A New Look at Love — 1978