Can I be happy?

I've been working at a department store for roughly the last month. Retrieving orders, ringing out and assisting customers and (mostly) straightening out clothes that have been picked over. Those moments when a customer wants something that I can't locate aside, it's generally less stressful than the last job...so why am I so annoyed with it?

I mean, it's an easy job and I make decent wages (and have an opportunity to do so past the holidays), but if I can't be happy doing a cake job like this, can I really be happy with anything?

What can I do to improve my disposition?