Wow! What a sentence! Let's break it down into parts:
"we tend to let our expectations sap the joy out of genuine experiences"
to tend to: This means we have a habit of doing this, it is something we regularly do.
expectations: what we expect will happen
to sap the joy out of something: this is a phrase meaning that something removes our pleasure from a certain experience (e.g. "My teacher is so boring - he really saps (or takes) the joy out of learning")
a genuine experience: an experience which is unique or somehow considered 'real'
So, if we put that together, we can see that the author thinks:
People have a habit of expecting things, so that when something genuine/real happens, they are so stuck on their expectations, that they can't really enjoy the situation.
An example of this would be when someone goes to a concert and expects to see fireworks. They go and there are no fireworks, but the rest of the concert is great. The person doesn't enjoy the concert because the whole time they think "Will there be fireworks? Where are the fireworks? I want fireworks. I'm so disappointed there weren't fireworks." Even if the concert was amazing, their expectations have removed their ability to just enjoy the event!