Stay Away From David
Sabrina's father warns her away from David because he's too far above her station. But you think he'd warn her away from David because David is an untrustworthy, irresponsible, woman-chasing doofus.
David "went through several of the best Eastern colleges for short periods of time, and through several marriages for even shorter periods of time," the voiceover narration says at the beginning of the film. He's attractive (William Holden is a super-handsome guy) but that's about the extent of his virtues.
It's true that David says he's in love with Sabrina—but how seriously are you supposed to take that? David is always falling in love with a pretty face or pretty legs—"How do you like those legs now?" he enthuses to Linus about Sabrina.
That doesn't sound like he's really in love with Sabrina; it sounds like he just likes chasing girls… which is perhaps why he ended up getting married and then divorced three times. Would his marriage to Sabrina last any longer? How long would David be faithful? Linus saved Sabrina from what would almost surely have been a pretty infidelity-infused marriage.
Poor David (Not Literally, Of Course)
Even if David is untrustworthy, you have to feel bad for him by the end of the film.
Maybe not marrying Sabrina was for the best. But probably marrying Elizabeth Tyson isn't going to go well. At least David likes Sabrina; he seems to hate Elizabeth, and he certainly doesn't think she loves him.
When asked what she'll do when she finds the engagement is ended, he says,
DAVID: Elizabeth will be so broken up she'll buy three new hats.
He thinks Elizabeth is shallow and superficial, and he doesn't think she really cares about him.
And yet, even though it makes it clear that David and Elizabeth are going to be miserable, the film is set to marry them off to each other at the end anyway. Linus and Sabrina get true happiness in Paris; David and Elizabeth stay in Long Island, consolidate the business deal, and are horribly unhappy.
David's a loser, maybe, but he's a charming loser. He cares about his brother and Sabrina; he wants them to be happy, and he steps aside so they can be happy:
DAVID: Funniest thing. Linus Larrabee, the man who doesn't burn, doesn't scorch, doesn't melt... suddenly throws a twenty million dollar deal out the window. [stops at the door]
DAVID: Are you sure you don't want to go with her?
LINUS: Why should I want to go with her?
DAVID: Because you're in love with her.
And for that gallant act, he's condemned to misery and woe. No kind act ever goes unpunished—especially if you're not in the leading role.