I believe that a lot of people give up on their dogs for casual reasons (can't be bothered, didn't put any thought into getting one in the first place, can't cope because they haven't bothered to train etc) but give excuses so they don't have to admit this - the usual ones being relationship split, house move / landlord, baby or allergies.
However - there ARE genuine cases and IMO it is unfair and downright rude / cruel to judge people without knowing the full circumstances. Even if you have been in a similar situation it wouldn't be exactly the same as various factors will come into play.
If a split leaves you homeless you may have very limited options - maybe moving from a normal house+garden to a third floor studio flat; or having to rent (which can be difficult with dogs); or having to get a room at someone elses place etc.
The split may have financial consequences - a dog that is easily afforded by a couple may be a struggle for a single person to care for properly - especially if the person keeping the dog has the lower income.
With the financial aspect comes work - a dog that once had compay most of the day may now have to be left for anything up to 12 hours a day, and hiring someone to help out might not be an affordable option at this time.
Emotional issues - some people end up in a serious state of depression, leaving them unable to care for themselves properly let alone another "dependant". Of course, for many people having the dog there will help - but we all react differently and some people may find the opposite happens.
Abuse cases I hope speak for themselves. If a person can't take their dog to a refuge and won't leave it with a violent partner....
In cases like these rehoming is done for the benefit of the dog.
How often on this forum do people complain about dogs being kept in unsuitable accomodation, left for long periods, etc. How do people react when someone comes on here saying they can't afford vet fees or a trainer?
You can't have it both ways.... it may simply be the case that rehoming is the best option, where the dog would suffer if it was kept.
Don't get me wrong, I find it heartbreaking reading all the ads and knowing many of the reasons given are nothing more than excuses...
But to sit here and slate people who may be having serious problems and are doing whats right by the dog despite causing themselves horrendous guilt and misery in the process I find pretty damn offensive.