In the first line of the poem, the speaker sets up an important contrast between himself and his beloved. Using a simile, he says that his beloved is "like to ice"—presumably meaning that she's cold, distant, and unwilling to "warm" to his love. By contrast, the speaker compares himself to "fire." This suggests that the speaker is burning with desire for his beloved, consumed with lust and passion.
The two are opposites then, at least in terms of their attitudes toward each other. And by ending the first line with a colon, the speaker presents this opposition as the crux of the poem: the fact that he and his beloved are as different as "fire" and "ice" is what gives this sonnet its central conceit.
The speaker spends the following lines trying to understand how "ice" can make "fire" burn hotter—in other words, how his beloved's disinterest only makes his own desire stronger. How is it possible, he asks, that his "so hot desire" doesn't simply melt her "cold so great"? Note the repetition of the word "so" here, which emphasizes the division between the speaker and his beloved: her feelings are just as cold as his own are hot.
The speaker is clearly frustrated by this, expressing that he is "entreating" his beloved, or begging her, even though she only grows "harder" in her resolve, like ice growing harder as temperatures drop. By framing his thoughts as a rhetorical question, the speaker further captures his vexation and his amazement at his beloved's enduring coldness.