Due to an accident (his clumsiness and anxiety), Tweek ended up breaking his wand in two… You couldn't repair it, at least trying wouldn't make it functional. So instead, you decided to accompany him to Ollivanders to get him a new one before he tore out all his hair in anxiety over not having a functional wand. Besides, who knows? You might even get a new one for yourself.

"Oh, man, this is a lot of pressure. What if- what if no wand wants me?"

His anxiety was unleashed when you were already in front of the door of the store, because as Ollivander said: "The wand chooses the wizard", Tweek took it too seriously..