I know people shouldn't get fat in the first place, but it ain't exactly a bad thing if you turn your life around and work to become healthy again and actually achieve it. Not an easy feat if you aren't in the right frame of mind. 

to inspire other fat people.....

the obesity just inflated so much they start publishing people losing weight so other people have a mentality that they will also be praised if they lose weight....

it is the same when you tell your wife or gf that they are losing weight (if theyve been working on it like diet or exercise.)

we want to remind ourself that it is possible... that they are not alone.... that they can do it.

it is not that bad.

why do we want to put ex-drug addict to speak? we should not give him a credit if that is the concept...

but they overcome soemthing... and we want to spread the word.

cultural behavioral conditioning.
Sure we don't praise the people who stay thin and look great. We just have sex with them instead. I think both have their positives.

You seem to be basing this on the idea that all overweight people were trim growing up and came from healthy environments. That really isn't true. Many of them came from situations where they weren't given healthy diets or, at the least, learned some very bad habits.

As losing weight really is extremely hard, I think anyone who has overcome being raised in an unhealthy environment deserves a lot of praise.

So millions of fattys this: She did it.. I can do that to!

And billion of dollars are spend into the healthindustry because of that..

It's because we're so fat.

Amusing OP. As to the reason why, pretty much all replies in this thread have covered that. It is hard to go back from addiction or in this case from being obese to physically fit.

Congratulating them acts as verbal positive reinforcement.

Because we want to show their horrid overweight image in TV and embarass them remembering how discusting they were.

Showing desirable women pertain to different show and we praise them with erections, nothing is more flattering than that to a woman.

And Who care about fit men? Not me.

To encourage others I suppose.

When the XB1 gets its first redesign and loses weight and that spare tire known as the external power brick, it will be greatly praised.

Its slimmer cousin the PS4 on the other hand, will go largely ignored when it becomes only slightly thinner and smaller than before.