The sweetness, mercy, majesty, And glories of my King; When I shall voice aloud how good He is, how great should be, (19-22)
The litany of adjectives here—"sweetness, mercy, majesty," "glories," "good," "great"—suggests that the speaker's feelings for his king are very strong. While he doesn't call it love, it definitely seems like he feels a lot of affection for his liege. The best evidence? Well, it's this very poem, of course!