In the poem "When I Heard the Learn'd Astronomer" by Walt Whitman, the author contrasts an analytical method of looking at the stars with a more naturalistic way of understanding them. The unnamed narrator, presumably Whitman, is attending a lecture on astronomy given by a renowned scientist. The speaker presents information on the stars in the form of charts, diagrams, proofs, and figures. When he has finished his presentation, his listeners are impressed and offer him applause.
The lecture does not have this effect on the poet. Rather than being impressed and invigorated, he feels disturbed, tired, and physically upset. This is because the poet does not see the stars as things to be objectively studied. To him, they are sources of inspiration. When he goes outside in the "mystical moist night-air" and observes the beauty of the stars "in perfect silence," his serenity returns.
In answer to the first of your questions, it is important to note that the poet does not complain about the lecture. He merely remarks on how it affects him personally. It leaves him mentally and physically uneasy, and so he goes outside. For him the stars are not objects to be studied but profound sources of inspiration. He does not derive strength from an analysis of their scientific properties, but rather from their beauty.
As for complaining about Whitman's poem, as mentioned above, Whitman does not complain. It merely express his preference to observe the stars rather than scientifically study them. In the same way, readers who are dissatisfied with Whitman's observations about stars could stop reading the poem and turn instead to a scientific text about stars if that is their preference.
Whitman's poem is not so much a lecture as an invitation. He offers an alternative to the prevalent scientific mode of analyzing the material world. He instead invites readers to draw spiritual inspiration from it.
The poet reaches out through his words to embrace his readers in the sense that he offers them an intimate glimpse into his own source of inspiration. He invites readers to join him in profound spiritual contemplation of the natural world.