The first three lines of "The Sun Rising" establish the relationship and tension among the three entities of the poem: the speaker, his lover, and the sun. The speaker disparagingly personifies the sun as a "busy old fool" who is "unruly" in the face of some authority. That authority is revealed at the end of line three to be "us," the speaker and an unknown party (later revealed to be his lover), who together relish the peaceful darkness of a curtained room.
Note the speaker's use of "thou" to address the sun. "Thee" and "thou" are not simply old-timey or high-brow ways to say "you." In Donne's day, common English distinguished between familiar and formal forms of the singular "you." "Thou" was an appropriate address for family members, close friends, inferiors, or younger people. "You" was actually more formal and polite, and it would have been the proper address for someone deserving of respect or reverence. By calling the sun "thou," especially when the more obviously intimate relationship at hand is that between the speaker and his lover rather than the speaker and the sun, the speaker insinuates that the sun is an inferior being who owes the speaker respect and obedience. By shining through the windows and curtains, the sun is being unruly and rude not to some unspecified authority, but directly to the speaker and his lover.
The two main poetic devices at play in these lines further serve to elevate the speaker's power over the sun. The speaker uses apostrophe to ask a rhetorical question of an entity, the sun, who can't respond. When the speaker demands to know why the sun insists on shining through the curtains, he is uninterested in an actual justification. Instead, the speaker wants to convince the sun not to "call on" the lovers to get out of bed by claiming that to do so is "unruly." Because the question (and indeed, the whole poem) is addressed to an inanimate entity (the sun), the speaker does not bear the same risk that other rhetorical questioners may face. That is, the sun has no chance of responding dismissively or returning a counter question. Through the combination of apostrophe and a rhetorical questioning, the speaker thus creates a platform for himself to lecture the sun at length, with no insubordinate interruptions.