I don't have to "struggle" to gain weight, but I have had cases where I need to. My ideal weight is about 185, give or take.
A couple years ago I was about 155-160. My eyes were sunken in, veins were extremely visible, you could count my ribs through my shirt. I would get out of breath quickly. I could sleep for 16 hours without issues. I was miserable. At the time I had no apetite for food, was working 16 hours a day 6 days a week, and had just lost my 1 year old puppy, Priscilla.
Before that I've also been 220lb. Ick, I hated it. Out of breath quickly, my body felt heavy, and I felt bogged down. It was awful.
Underweight or overweight sucks, and I sympathize with those who have difficulty. The problem that a lot of overweight or healthy people have with those trying to gain weight is its not about gaining x amount of pounds. Its about gaining healthy weight, not just fat. So it requires eating and exercising right just as much as losing weight does.
That's why I don't worry about weight. I just ask myself, am I healthy enough to do what I want? And that works for me. By the time I am hitting 190-195 that answer becomes "no", same with hitting 170-175. So as long as I'm in between I don't mind it too much.