Why do people automatically assume it's better to be 170 pounds than 220 pounds? Being lighter isn't always better than being heavier ?

It depends on height, for sure, but generally lighter is better as with so many things in the world. 

It isn't necessarily better but it is in the vast majority of cases.   Body fat percentage is a better measure but most people don't really need that, it's obvious.

Because being overweight is unhealthy. it puts a strain on your heart. 

Weight is the WORST metric of health.   What matters is fat.  Fat doesn't even weigh that much. 

No target body weights; waste of energy and time. 

Losing excess FAT is the goal.  Dieting CREATES fat, FYI.

I listed the answer below.   Dieting ALWAYS fails,  exercise has almost nothing to do with significant fat loss or gain. 

But there IS a simple solution. 

Being more than 10% underweight is as unhealthy as being 10% overweight.

Of course you would need to be a male over 6'7"  for 220 to be considered a healthy weight, without extenuating circumstances.

As much as people do not like to admit it, there's certainly a sigma in society where you're more desirable if you're more in shape. At 220lbs, most people would develop a belly, a muffin top, back rolls, and/or a chubby face. At 170, for some, none of these would be existent and finding clothes that fit just right wouldn't be much of a problem either.

It would depend entirely on the person's height and skeletal mass.   And being overweight isn't healthy

TROLL!!! You asked the same very question the other week.

Stop feeding this TROLL!!

Actually i was surprised that my older doctor actually prescribes to the idea now of 'being healthy overweight' .As long as you don't have diabetes issues or other issues .