Psalm 100 says:
עִבְד֣וּ אֶת־יְהוָ֣ה בְּשִׂמְחָ֑ה
Serve God with joy.
Psalm 2 says:
עִבְד֣וּ אֶת־יְהוָ֣ה בְּיִרְאָ֑ה וְ֝גִ֗ילוּ בִּרְעָדָֽה
Serve God with awe/fear; tremble with fright.
How can one do both at the same time?
I suppose one can do both, but it's rare. In my case, as a physicist, I always experienced quite a thrill when I saw that the most complicated things in the world derive from simple equations, simple principles. Understanding the laws of physics and solving intricate problems always had me in awe and made me appreciate how "smart" God is. This is what Einstein called “cosmic religious feeling”. It has to be experienced to be understood. The awe and the joy were indeed there at the same time. However, (1) there was no fear, and (2) I don't see how I was "serving God".