If you are aware of only the Physical aspects of your service of Hashem, you might not be so happy. The physical without spiritual awareness can become dry.  

     For instance, if you are learning Torah and are not aware of Hashem's presence, you might think that the Torah is only talking about our history, about the Jews who lived way back then, and not about us! But, "All of the deeds of our fathers are signs to us in our day!" Try to see how that Torah portion you are learning applies to you.

     Or, if you are doing the mitzvahs, and seek to only satisfy their physical requirements, and you do not think of the holiness they are bringing into your life, they could seem to you to be a burden, G-d forbid.

     Hashem gave us His Torah to relieve our burdens, not to increase them.