Explain the meaning of "Methinks, mistress, you should have . . . I can gleek upon occasion."

This quote comes from act 3, scene 1. In this scene, the mechanicals discuss how they will perform Pyramus and Thisbe.
When the rehearsal begins, Puck sneaks in and casts a spell on Bottom. Now, Bottom has a donkey's head on top of his shoulders (where his own head should be). Meanwhile, Quince and Snout try to tell Bottom that he has been transformed. The latter refuses to believe it and begins singing wholeheartedly.
It is Bottom's singing that wakes up the sleeping Titania. When Titania's eyes alight on Bottom, she instantly falls in love with him. She tells him that she is enamored by his singing and absolutely entranced by the beauty of his figure. It's clear that Titania is under a spell.
For his part, Bottom answers:

Methinks, mistress, you should have little reason for that. And yet, to say the truth, reason and love keep little company together nowadays. The more the pity that some honest neighbors will not make them friends. Nay, I can gleek upon occasion.

What Bottom is saying here is that he doesn't believe Titania has any reason to love him. He maintains that it's difficult to find reason accompanying love in today's age. Bottom's words are true in the sense that romantic love often defies logic: each person differs in what he/she admires in a lover. In fact, the saying "there is no accounting for taste" holds true here.
Bottom proceeds to argue that there is a need for honesty; after all, love should be inspired by reason, not mere emotion. Bottom then ends his little speech by insisting that he's just joking. The word "gleek" refers to joking or jesting. However, Bottom's words are significant because they truly describe Titania's present state of mind. Being under the influence of Oberon's spell, she can only make foolish decisions when it comes to love. Oberon had originally intended for Titania to fall in love with the first creature she saw.
Essentially, Oberon wants to embarrass Titania because she refuses to allow her beloved attendant to serve Oberon. The spell is Oberon's plan for distracting Titania while he schemes to get what he wants. So, Bottom's words may be spoken in jest, but their meaning is significant. Shakespeare often uses humor to relay important truths to his audience.