In regards to why it's possible, I'll just echo what TheOtherDaveSaid.

The reason it's helpful to try for a single top-level utility function is because otherwise, whenever there's a conflict among the many many things we value, we'd have no good way to consistently resolve it. If one aspect of your mind wants excitement, and another wants security, what should you do when you have to choose between the two?

Is quitting your job a good idea or not? Is going rock climbing instead of staying at home reading this weekend a good idea or not? Different parts of your mind will have different opinions on these subjects. Without a final arbiter to weigh their suggestions and consider how important comfort and security are relative to each other, how do you do decide in a non-arbitrary way?

So I guess it comes down to: how important is it to you that your values are self-consistent?

More discussion (and a lot of controversy on whether the whole notion actually is a good idea) here.