So when the singer, 48, and the fashion icon, 38, broke the news with PEOPLE on Sunday that they're separating, friends were caught off guard.
"All this is just shocking to me," says Niche Media founder Jason Binn, a friend of Klum's for more than 20 years. "I've only seen them as the most loving couple."
"The Heidi and Seal I knew was the couple everyone loves," says a fashion industry source who's worked with Klum, "super passionate and in love."
Sources who've witnessed Seal and Klum's typically overt shows of affection – "They were always all over each other, practically groping," says another fashion source, "totally in love" – are now left wondering what could have gone wrong for the pair, whose children are Leni, 7½, Henry, 6, Johan, 5, and Lou, 2.
"It has to be something major," says a Klum friend. "They have four kids together, and they've always been very committed to making it work."
With reporting by CHARLOTTE TRIGGS and ULRICA WIHLBORG