Beagles howl because they are hunting dogs and do so as a reaction to certain stimuli or to express themselves. To explain it in the simplest way, beagles like to howl.
As hunting hounds, beagles howl in order to inform the hunting party about a successful hunt or kill. Beagles are easily distracted by other animals and sometimes may howl while chasing one, which is why they need to be kept in a fenced yard for exercise. However, as a pet, there could be a number of other reasons why beagles would go on howling, but it is mainly to express themselves or in order to catch other’s attention. It is also sometimes an expression of excitement and when they have discovered something of interest. Beagles may also howl because of stress, anxiety and boredom. Not all beagles may howl though.
For those who are looking to keep beagle as a low key pet, their howling could prove to be a huge problem, which is why this breed of dogs is not for everyone. Howling beagles can particularly be irritating for people living in connected apartments and closed building complexes. Beagles usually are pretty stubborn and single minded and unless the owner knows and has experience to handle beagles, their habit could become troublesome. But it is largely a matter of personal preference, as some people may find their howling habit cute. Furthermore, their howling habit also makes them a good watchdog.
In short, howling beagles crave attention, work and play and if you are not offering enough, expect more of it. Beagles are also dogs who love to spend time with people, so they are most likely to get distressed when they are alone and sometimes even suffer separation anxiety when separated from those of their kind. Perhaps keeping the busy could be one way to control a beagle apparently howling for no reason.