I was always underweight & was obsessed with gaining muscles. Is it healthier to remain thin rather than force

I think it comes down to lifestyle, its a personal decision. I'm quite a bit taller than you and when i started to workout i focused only on chest and arms. It was horrible I had the worst back pain to the point where it hurt to have even close to proper posture. at 6' and 155 lbs you should be fine in terms of getting sick etc so the choice if yours. Personally I stopped focusing on arms and started to do a lot more squats and dead lifts and strengthened my back muscles. It still hurts a bit but its definitely manageable now.