Self-loathing people (those who experience intense feelings of inadequacy) are characterized by intensely negative thinking about themselves, in particular their talents and abilities, their self-worth, and their ability to be loved. (As you may know, this is also commonly associated with depression, from which I am sure many such people suffer.) I've written here before about how hard it can be for someone who loves a self-loathing person to cope with this. Another thing that puts stress on such relationships is the self-loathing person's projection of the negative attitudes he feels towards himself onto what other people say to him.
It is a fact of language that it's vague—any statement can be interpreted an infinite number of ways, even when intonation and tone are taken into account. A seemingly innocuous statement like "isn't the sky blue today" can be taken a number of different ways depending on how a person chooses to hear it. It could be an expression of joy, a statement of appreciation for the beauty of nature, or a snide comment on the mood of the person to whom it is said ("What, are you implying I should be grateful for the refraction of the sun's rays through the atmosphere? Oh, thank you Mr. Sky!").
OK, maybe that last one was a bit extreme, but likely not unfamiliar. (Double negatives rock.) A person who persistently thinks negatively about himself or herself may also assume that others think the same way, no matter how strenuously they deny it. So if a self-loathing person regards himself as gloomy, he may project that onto others, assuming they find him ungrateful—and voila, thank you Mr. Sky. (Now, Mr. Sky, a word about all this $%^& snow...)
Likewise, if the self-loathing person thinks of himself as stupid, and someone ribs him about a minor slip-up he made that day, he may take that as serious criticism rather than an innocent joke. Of course, the other person didn't mean it that way, but the self-loathing person hears it that way nonetheless—because that's how he thinks of himself, so that's how he assumes others (really) see him too.
With respect to relationships, many self-loathing people feel unworthy of their significant others' affection. So, any criticism, no matter how slight or humorously intended, may be taken to the extreme, and lead the self-loathing person to think, "OK, so now we're in agreement—I suck." Does this mean, if you're in a relationship with a self-loathing person, you can never criticize him or her, and instead you must provide constant coddling and positive reinforcement? Of course not—no one should be immune from criticism, even if they're naturally disposed to take it harder than intended.
One problem that self-loathing people often have is that they place too much emphasis on negative reinforcement (criticism) and too little on positive reinforcement (praise). Praise is assumed to be insincere or undeserved, while criticism is assumed to be understated and very deserved, confirming the person's own beliefs about himself. So lavish as much praise of them as you can, but they'll still only remember when you told them they put too much sugar in your coffee, which in their mind confirms that you think just as little of them as they do.
For the partners of self-loathing people, I would simply advise not that praise be given more often, but that you make sure the other person listens to it, really hears it, and understands how sincere it is. By the same token, you don't have to criticize them less often, but perhaps make an effort to ensure that it is taken only in the spirit intended—too much sugar is just too much sugar, not a death knell for the relationship.
To the self-loathing person, I would say that regardless of how you feel about yourself, you should give other people the benefit of the doubt that they are sincere when they praise you. By the same token, when others criticize you, try to hear just what they say, no more. It is all too easy to read too much into what people say, especially when you impute your own feelings of inadequacy onto their words. But that is your problem, not theirs—particularly when the other person only wants to love you.
You may not understand why he or she loves you, but you don't have to—just stop questioning it, start trusting in it, and learn to appreciate it. (And thank you, Mr. Sky.)