Is "because it's fun" an acceptable justification for choosing a certain research path (when communicating to peers/colleagues)? When asking a peer or colleague why they are pursuing a certain research direction, I have often heard the answer "because it's fun". This bothers me: Shouldn't we all be capable of providing an honest narrative explaining why our chosen research direction is of value (other than personal pleasure)?

To clarify my question: I think it's fine if "fun" is an intrinsic motivation for choosing academia as a career, I also think this is necessary to success. I also don't think we should be forced into a certain definition of “useful research”. The concept of usefulness or value can differ per individuals. But shouldn't one have a strong explanation for the value of one's research other than one’s own enjoyment?

Note: I am working on fundamental research in quantum physics.
There is a difference between the personal motivation of a person to work on a subject, and motivating why a subject is worth working on. We should always be able justify the later, both from an ethical and practical point of view ("because it is fun" will not convince anybody to pay you to do it). 

In practice most researchers (certainly the successful ones), can provide a detailed motivation of why their work is relevant to the furtherment of their field and humanity as a whole.

However, this motivation might differ quite significantly from their personal reason(s) for wanting to work on the subject. In particular, they typically will have reasons for working on this specific thing, rather the long list of other things that are also worth doing. (In this category "because it's fun" is probably preferable over  the much more cynical "because we can".)