Brochas: Daf Lamed amud beis

Mishna: One should not rise to pray (shemone esre) other than with an attitude of "coved rosh" (seriosness/reverence).

Gemara: From where (do we know) this?  ...R. Nachman bar Yitzhak said: Serve Hashem with "yira" (awe), and "gilo" (rejoice) with trepidation (Psalms 2:11)

The Bach (Orach Chaim 93)interprets the Gemara as suggesting that the preferable state of mind is awe. Failing that, one should have "simcha" (joy) in the mitzva of avodah/tefilloh, but should be careful to avoid "s'chok" (levity).

The Maharal (Netzach Yisrael 23) prefers an approach which suggests that there is no dichotomy between reverence and joy when it comes to tefillah.

The requirement of reverence is a function of Chorban Beis haMikdash, the destruction of the Temple. We need to recognise that the world has lost something fundamental and that unless there is a serious recognition of this fact, we cannot legitimately aspire to a restitution of the status quo ante. As such, the quality of our current interaction with Hashem which has become flawed due to the loss of the Beis haMikdash, (as it says on daf 32b: From the day the Temple was destroyed, the gates of prayer were locked) is dependent upon a certain seriousness/reverence of approach, facilitated by a recognition that we are created beings addressing our creator, an awareness of which should inspire the appropriate level of awe.

At the same time, a feeling of joy is completely appropriate. It stems from an understanding based on precisely the same factor that inspired awe, namely that we are created beings addressing our creator. The analogy the Maharal cites is of a servant addressing his master. And of course, there exists the strongest of bonds in this master/servant relationship.  We should be happy that as a servant we have automatic access to our Master and the ability to communicate to Him our needs and aspirations.