Shortly after, Willows character changes as she starts to use magic for non-essential purposes. For example: Willow conjures up confetti to decorate Buffy’s house with (after Anya and Xander announce their engagement), raising concern from both Giles and Tara (as she could have just gone to the local store to purchase some).
Willows character then undergoes a more serious change, as she starts to use magic to make things the way she wants them to be (without regard for others). For example: Willow deliberately casts a spell on Tara to make her forget about an argument that they have had (ironically over Willow using too much magic). When Tara discovers the truth, she gives Willow a stern warning that “you know how powerful/dangerous magic is” and that “too many spells is not good for you Willow”, eventually leading to Willow agreeing to go “cold turkey” for a week (i.e. not cast any spells). Unfortunately, Willow is so addicted to magic, that she casts a spell almost immediately, and when Tara finds out, she leaves Willow.