D'Artagnan has lost every person he loved (his mother, father maybe also a boyfriend or a girlfriend) so he started to think, that he is cursed.
When he meet the three Musketeers, D'Artagnan is afraid that because of him they will die too, so he tries to be rude, unpleasant and doing everything to stay away from them.
At the beginning Athos, Porthos and Aramis are shocked by the boy's behavior, but finally they understand that there's something he hides under the guise of bad manners.
D'Artagnan's curse