Most people who dress up their bullmastiffs do so for one of two reasons.
The first reason is because the people find it funny.
Let's face it, a bullmastiff wearing clothes is cute.
It's a great topic for conversation.
It's fun at holidays and parties.
However, it's not necessarily as enjoyable for the bullmastiff.
Bullmastiffs are individuals, just like human beings.
Some bullmastiffs may tolerate dressing up better than other bullmastiffs do, especially if there is food or some reward involved, but that does not necessarily mean that they like it.
The second reason people dress up their pets is for protection from the cold.
This reason is completely valid for certain breeds.
For instance, breeds such as the chihuahua originated in warmer climates.
Therefore, if you own a dog that is native to a warmer climate and you live in a colder climate they have trouble keeping warm in such weather.
In these cases sometimes a dog sweater is essential.
It is very important, no matter what the reason for dressing up the bullmastiff, to supervise the bullmastiff if it is dressed up.
Bullmastiffs shouldn't be dressed up in anything that they could easily get tangled in or chew up and choke on.
If the bullmastiff does not take well to being dressed up it could easily start running around trying to escape and get caught on doors or other objects.
The bottom line is that if the bullmastiff needs clothes to go outside in the cold or seems to tolerate them OK at holidays or special occasions then there's no harm done.
However, if the bullmastiff doesn't need clothes and doesn't seem to like them then they shouldn't be forced into it.
Bullmastiffs, like people, don't usually like to be forced to do something for someone else's amusement.