This article takes on this subject from an angle that I've never really considered before (well, I actually wrote the article in question, but before I wrote THAT I hadn't considered it!) Usually when you ponder extra workload and demanding bosses, you believe it to have a negative connotation, but actually, this could be happening for a far more positive reason than you might think. 

In some instances, if the boss is giving you extra work and involving you in cross-functional projects, it kind of means that:

A: They like you 

B: They trust you 

C: You're good at your job

Of course there could be many situations where this may not be the case. For example, if you work for a small company, how do you really know that the extra workload is in your best interests or if it's just abuse of your time and (possibly cheaper) labour? 

Have a read: