"The Canonization" begins with an explosion of frustration. Without preamble, the speaker bursts out: "For God's sake hold your tongue, and let me love." Slapped awake like this, readers are primed to figure out what on earth could have gotten the speaker so touchy.
Some friend or other, clearly, has been teasing the speaker about being in love. So far, so normal: mocking lovers is a timeless hobby, popular for as long as lovers have existed. But the speaker's next lines hint that his friend isn't just making fun of him for being all infatuated. So long as his friend "let[s him] love," the speaker says, it doesn't bother him one bit if they want to mock him for:
[...] my palsy, or my gout,
My five gray hairs, or ruined fortune [...]
With "palsy" (shaky hands), "gout" (creaky joints), and "five gray hairs" (which explain themselves), this speaker is no spring chicken. And not only is he older than he used to be, but he's poorer, too: he once had a "fortune," but it's gone now. (Perhaps he's even down on his luck in other ways: a "fortune" can mean both one's wealth and one's fate.)
In other words, this speaker is a middle-aged man who's both down-at-heel and head-over-heels. And who's more mockable than that? Passionate love is often considered the purview of the young and silly; older people who fall hard for each other, in Donne's 17th century as much as now, are easy to accuse of midlife crises or plain foolishness.
But this speaker stands ready to defend himself against such charges. Sure, he's old, and sure, his purse is a little lighter than it used to be—but he doesn't mind being teased about any of that, so long as he can go on being in love. Love, in other words, means he doesn't care one bit about anything else. This poem will celebrate passion, whenever and wherever passion arises.