The main reason why Alceste would want to marry a woman with as many flaws as Celimene is because, as it is seen toward the end of the play, Alceste ultimately understands that nobody, himself included, is perfect.
It is true that Celimene is a gossip, judgemental, and insulting. Yet, when we perform a close reading of her character, we realize that Alceste's over-exaggerated sense of honor and integrity are as damaging to himself as Celimene's flaws are detrimental to her own character. In all, Celimene and Alceste balance each other out.
Aside from what could be consiered "personal" motivation on Alceste's part, consider Moliere's purpose in creating his character: Alceste is supposed to be an outcast; someone so out of touch with what is really going on around him that he encases himself in a personal "bubble". Yet, in doing this, Alceste makes himself vulnerable and open to the criticism of others. In Philinte's own words:
The failings of human nature in this life give us opportunities for exercising our philosophy, which is the best use we can put our virtues to. If all men were righteous, all hearts true and frank and loyal, what purpose would most of our virtues serve?
In other words, becoming a misanthrope due to an extreme sense of virtue is just as bad as exposing our imperfections to the world. In his love for Celimene, Alceste demonstrates that he (perhaps secretly) does concede as much. In her flaws, Celimene keeps Alceste closer to reality.