Is being underweight bad?

Their are certain phenotypes that exhibit this behavior (they call it "hard gainers" in the magazines and circles) because it's in the individuals genotype coding mostly for this phenotype. While the other side of things, those who can put on weight, it tends to be more reversed. It's genetics, metabolism, and biology (sometimes environment too).

You say things, "I have to force feed", this is common to hear and makes sense. This is because perhaps you are more sensitivities to the effects of certain hunger hormones like gherlin or GIP, or perhaps it's brain and regions that controls feeding? Maybe it's sensitivity for certain neurons such as NPY/AgRP neurons or maybe resistance at the PCOM regions. It could even be deeper and not regulated by feeding directly, but indirectly with hormonal sensitivity of leptin to these neurons and how they responds to energy homeostasis over all!

The point is if some people have the biology that keeps them under feeding and or lean, of course you will have trouble when biology is working against you.

This is why drugs work - they step oud side normal physiology. This also explains why fat people stay fat (don't get fat, and you don't cause the metabolic defects of expressing certain genes that keep you fat)

Side note. Here is something to observe in the U.S. - Notice the doctors prescribing medicine that influence our neurotransmitter heavily involved in feeding regulation such as serotonin, dopamaine, etc? Now notice the majority of mental health population end up being over weight? There is a correlation to that. It also isn't safe to conclude that is the answer to over feeding, for example certain atypicals can cause insulin resistance regardless and those who are lean can still develop T2D.

The humor comes when those people are already over weight and fat and are now told to simply exercise as if they can beat the body's physiology working against them (their obesity genes are expressed and pathological conditions in their biology as been set). I never understood the logic here and so has hasn't obesity researcher who keep calling this practice of medicine into question.

The point of that last part is to give you an idea how complex energy regulation is and why we have weight issues and problems. Politics, genes, metabolism and simply biology.

I should also point out that those with this type of biology tend to metabolize (catabolize) muscle tissue at a faster rate, therefore effect cellular turnover, thus keeping protein in muscle would be harder to do for these people. That is assuming that they can even keep a diet that doesn't inhibit feeding in the first place (protein synthesis require more than animo acids, so does the biochemical pathways that build them - think calories)