Welcome to the NHK! fits this perfectly I think. Also, it is a very good manga in general.

For some reason... maybe Hana no Namae? Though the difficulties arise more because he is (kind of, sort of) mentally ill.

Seconding Boyfriend (Souryo Fuyumi). I have looked at the whole series (mostly RAWs) and it has aged quite well. 😀

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Kokou no Hito might not seem right at first glance, but he gets a girlfriend eventually. It takes a long time (about 10 vols?) for the story to get to that point, however.

Sangatsu no Lion and Billionaire Girl. Romance will (inevitably ^^) turn up at some point, though there are only hints as of now. A slight problem arises due to the fact that neither of the MCs are actually deliberate loners. They've become lonely by circumstances and various chances, and have since gotten used to it. They don't hate their isolation and desperately seek love, but they don't actively desire being alone either. But I think you should still read these, as you're sure to like them regardless.

Seconding NHK ni Youkoso! and Octave. I hesitate to second Oyasumi Punpun because as far as I can recall, the male lead often desires companionship, despite being incapable of breaking out of 'teen angst mode' long enough to properly grasp it. But I also admit that I stopped reading this ages ago, and could be completely wrong about that. Still, it's a very realistic manga - so you definitely should try it if you like harsh realism in your fiction (can't say I fancy it much, myself).

Hana wa Saku ka fits quite well. It's a very nice story, but not being seinen (unlike the above titles), it doesn't have the level of seriousness you seem to be looking for. And I bet you already know about this one, anyway.

One of the couples in Kanojotachi no X-Day. I personally loathe this manga, so don't consider it a recommendation.

Apologies if these don't fit perfectly (I'm only regularly following Kokou and HwSk); I'm not the most reliable source when it comes to love stories >.<