Is a publication with co-authors worth less that a sole-author paper? Does this explain people's reluctance to include someone as a co-author? Is there a lower value to publishing with other co-authors instead of just oneself? Why are some people so reluctant to acknowledge your participation and contribution?
This is, in my (admittedly limited) experience, heavily dependent on your field. 

In some fields, single-authorship is the norm; I believe a lot of highly-theoretical mathematics and some social sciences go this route. In other fields, such as astrophysics, this is almost never the case, as the work is often conducted among dozens of researchers working on multiple continents.

From what I can tell, publishing with co-authors tends to signify one or more of the following things:


The work is highly multidisciplinary (even within a single field) -- some research requires many experts in many different domains working together.
The work is "labor-intensive" -- some research requires a great deal of preparation that would be infeasible for a single researcher. For example, in my field of computer engineering, there's usually a significant amount of coding that goes into the research, which is most easily/quickly accomplished by many students and postdocs working together.
The work is large in scope -- larger, more ambitious projects require a higher level of support and more minds working together.
The work involves someone famous and everyone wants a piece of the pie -- okay, admittedly I made this one up.


That's not to say that single-author paper don't fit into one or more of these categories. These are just some rules of thumb that I've seen so far. 



As for why some people are reluctant to "acknowledge your participation and contribution": well, this ranges the gamut of everything from the evils of:


Narcissism
Ego
Desire for fame/prestige


to the more mundane/less evil problems of:


Disagreements on what constitutes a large enough contribution to claim authorship
Misunderstandings as to what is being contributed (by one or both parties)
Traditions on what is deemed worthy of acknowledgement in publication.


In short: human nature.

If you're concerned that you aren't being credited for your work, you need to speak with the other party ASAP and set out clear guidelines moving forward as to where credit will be given and for what. Get this in writing -- timestamped and signed preferably, email is great for this.