There are some words that are just really hard to hear. You're an adult and you can handle rejection, you can't be everyone's friend but you still like to make some kind of impression when you are around. See women have an odd way of referring to men that they don't really have an opinion on. "He's alright/nice/lovely" etc. etc. etc. They're all good terms on a normal occasion, but when they refer to someone that doesn't excite them in any way at all it has this hollow sound to it. Don't get me wrong, men use the same terms in the same way. But I don't know the word that makes women feel like sprouts on Christmas... whereas for men is 'Harmless'.
1. There's not much to you.
Harmless should be a good thing right? If you are harmless then you aren't a threat and that should be good to hear. Nobody likes to be around a threatening person, it's just not comfortable to be around someone that makes you feel as if they might shank you like a prison bitch without a moment's notice. They're safe to the extent that you'd see them as a sibling. Nooott so good.
2. You don't make an impression, good or bad.
Harmless is being non-threatening in the same sense as the people that walk past you every day, you don't see them as a threat so your brain just ignores them and that's what really hurts with it. You're forgettable, you're a background character, an extra used to make the scene seem more full. Harmless can be used as a way of saying that they just bore you, that they blend in wherever you are like a piece of furniture. Actually more like wallpaper because people want to sit on furniture. No one wants to be known as that.
3. Harmless is just harmless.
There are dozens of terms that can be used to describe someone but they carry different meanings, mostly based on intonation. 'Cute' has two drastically different meaning depending on tone. One means that you're likable, charming, the other that you're like a little kid that doesn't get taken very seriously. Harmless is more like 'cuddly', nobody wants to be called cuddly. Teddy bears are cuddly and you are no bear, you are a strong, virile adult male. Or at least that's how you'd like to be seen.
4. It hits you deep.
It isn't all in reference to being seen as attractive or as masculine, those are totally subjective. To be honest I've never liked people that need to be the centre of attention all the time and I think that's a fairly common thought but it's still nice to be able to get command of an audience for a few minutes. You can recognize that you're a shy person and maybe you don't stand out to much, in these cases you'd be referred to as quiet or something of that like. Harmless means you stood out just enough to be recognized but not enough to be noticed. Ouch.
5. It's a good way to crush someone's spirit.
Say you have made a good first impression with someone you like and things went well. It's the kind of thing that can inflate your ego, maybe inspire a bit of confidence in yourself. But then you get referred to as harmless, especially by the person you've just been with and it can bring you crashing down. You didn't make much of an impression during and that's fine, you're an adult, you can accept when you were shite but it still doesn't do much to ease the idea.
6. A few other terms to use.
Sound, good fun, seem nice, all good terms, they pretty much mean the same thing and change with intonation but they have better associations. This isn't to say that you should refer to someone in a positive sense if you don't have an opinion but for the sake of the description you may as well just go with something on the nicer end.
Video: Nice Guys Do Not Finish Last