The sweetheart is the opposite of the jerk -- someone who very highly values the perspectives of others around him.
You might think that if being a jerk is bad, being a sweetheart is good. And I do think it's better, overall, to be a bit of a sweetheart if you can. But I'd also argue that it's possible to go too far toward the sweetheart side, overvaluing, or giving excessive weight to, the perspectives of others around you.
I see three moral and epistemic perils in being too much of a sweetheart.
First peril: The sweetheart risks being so attuned to others’ goals and interests that he is captured by them, losing track of his own priorities. Consider the person who never says “no” to others – who spends his whole day helping everyone else get their own things done, leaving insufficient time to relax or to satisfy his own long-term goals. The sweetheart might forget that he can also sometimes make his own demands. Sometimes you need to disappoint people. In the extreme, the sweetheart’s complicity in this arrangement becomes in fact a kind of moral failure – a failure of moral duty to a certain person who counts, who ought to be respected, who ought to be cut some slack and given a chance to flourish and discover independent ideals – I’m speaking here, of course, of the duties the sweetheart has to himself.
Second peril: Because the sweetheart has so much respect for the opinions of other people who might disagree with him, he can have trouble achieving sufficient intellectual independence. This is part of the reason that visionary moralists are often not sweethearts. The perfect sweetheart hates disagreeing with others, hates taking controversial stands, prefers the compromise position in which everyone gets to be at least partly right. But everyone is not always partly right. Southerners oppressing black people were not partly right. Physically abusive alcoholic husbands are not partly right. Some people need to be fought against, and the purest sweethearts tend not to have much stomach for the fight. Also, some people, even if not morally wrong, are just factually wrong, and sometimes we need a clear, confident, disagreeable voice to see this.
Third peril: To the extent being a jerk or sweetheart turns on how you react to the people around you, being too much of a sweetheart means risking being too captured by the perspectives of whoever happens to be around you – without, perhaps, enough counterbalancing weight on the interests and perspectives of more distant people. The homeless person right here in front of you might compel you so much that you set wrongly aside other obligations so that you can help her, or you give her money that would be more wisely and effectively given to (say) Oxfam. When you’re with your friends who are liberal you find yourself agreeing with all their liberal positions; when you’re with your friends who are conservative you find yourself agreeing with all their conservative positions. You are blown about by the winds.
If you know the cartoon SpongeBob SquarePants, the humor and conflict in the show often derives from SpongeBob's excessive sweetness in these three ways.
I’m not sure there’s a perfect Aristotelian golden mean here: an ideal spot on the spectrum from jerk to sweetheart. Maybe there’s one best way to be – partway toward the sweet side perhaps, but not all the way to doormat – but I’m more inclined to think that perfection is not even a conceivable thing, that one can’t be wholly true to oneself without sinning against others, that one can’t wholly satisfy the legitimate demands of others without sinning against oneself; that everyone is thus deficient in some ways.
Furthermore, when we try to correct, often we don’t even know what direction to go in. It’s characteristic of the sweetheart to worry that he has been too harsh or insistent when in fact what he really needs is to be more comfortable standing up for himself; it’s characteristic of the jerk to regret moments of softness and compromise.