Like many of Shakespeare's sonnets, Sonnet 75 has a pretty ambivalent view of love. Like, sure, it brings happiness (as you can see in the pleasant imagery of lines 1-2), but it can also make people swing violently from extreme euphoria to the depths of despair. Reading Shakespeare's book of sonnets as a whole will provide plenty of examples of both emotions. Sonnet 75 just happens to bring them both together. (We think it's actually a bit more on the despair side, because even the speaker's descriptions of himself when he's successful in love sound unpleasant, since they're excessive.) We don't know what Shakespeare's personal thoughts on love were, of course, but they certainly are the thoughts of the speaker of these poems.
One thing we do know about Shakespeare himself, though: the guy was positively exploding with poetic ideas. As in: ka-boom. Instead of letting depressing topics get him down, he just used it as a stimulus for greater creativity, as you can see in the way he cycles through different imagery in this poem (for more on that, check out our "Imagery" section).
Finally, another major giveaway is the "Shakespearean sonnet" form (we talk about this in greater detail in our section on "Form and Meter"). As Shakespearean sonnets go, of course, this one is unusual because it doesn't divide up its topics according to the units of the rhyme scheme (three four-line quatrains followed by a two-line couplet). In a weird way, though, this might be the most Shakespearean thing about the poem. Through his constant explorations of the sonnet form, the Bard seems, on several occasions, to want to break away from the poetic forms he has mastered.