Thanks for your guidance.
Aim: The Antidote for Sloth and Torpor
The jhānic factor of vitakka or aim has the specific power to open and refresh the mind. It makes the mind alive and open. Thus, when the mind is continually and diligently trying to be accurate in aiming at the object, sloth and torpor do not arise. A mind attacked by drowsiness is a mind that has been constricted and withered. Vitakka is the antidote to thina middha.
Often, it was joy that free my mind from drowsiness and restlessness.
Following your advice of doing walking meditation to counter drowsiness, yes it works on refreshing the mind but when i regulate to sitting meditation with a fresh mind, i rarely got the same joyful sensation which hit me when meditating during drowsy mind. Thus had a hard time dealing with restlessness. Like the title says, i still can't figure out why pleasant feeling arise more often when i was drowsy(not heavily), need more advice.
To be clear, i'm not being attached to pleasant feeling. It is just that i need it to deal with both drowsiness and restlessness. My mindfulness and concentration are not as intense without it during meditation.