Thomas Jefferson once claimed that if he could only go to heaven but with a party, he wouldn't go. Indeed the word "party" (at least in the context of a political party) never appears in the Constitution. Why was this?
Also, did they have a change of heart at some point, or did they simply create America's political parties out of practical necessity?
They saw them as potentially corrupting the system that they had set up to prevent any special interests (they included parties in this group) from gaining too much power, they eventually came to the conclusion that the only way to beat them was to have them work against each other.
As to the second part about creating parties later on, parties aren't sanctioned by the government or anything, they're really just private clubs that make their own rules on how they provide and select candidates for political offices.