Common wisdom suggests that it's a bad idea to go grocery shopping while hungry. If you haven't eaten in some time, chances are everything in the store will look good, and you might end up wasting money by making frivolous purchase you wouldn't normally make. Hunger clouds your better judgment, and you could end up with a lot of ingredients that add up to nothing, or with several precooked 'quick fix' items that you would have been better off leaving behind.
The same logic applies to shopping for new clothes, but this fact is far less accepted. When people are in a bad mood, they occasionally head to the mall to make themselves feel better, a phenomenon sometimes called 'retail therapy'. Retail therapy may work in the short term, but in the long run, engaging in this practice results in losing money by making impulsive buying decisions and spending irrationally. As a general rule, then, it's better to go shopping in a good mood.
When people feel bad about themselves, they want to surround themselves with items that will make them feel better, whether about their physical appearance, social stature, or romantic prospects. Sometimes, a change is required to get out of a bad mood, and while it's important to recognize this, a change in wardrobe probably will not ultimately do the trick. Although we might feel compelled to buy clothes that convey a different personality than the clothes we usually wear, the end result of such impulse buys could accomplish the opposite result.
New clothes, if chosen wisely, can inspire confidence, and this could be the motive that drives some people to engage in retail therapy. As with hunger, however, negative emotions can cloud our usual, rational judgment, and cause us to buy things that, in a better mood, we would never have considered. If you are passed up for a promotion at work, you may think that buying a few items will spruce up your appearance and make you more impressive, giving you an edge next time around. This could be true, but visiting the department store on the way home from work will probably not solve the problem.
Your Unique Style
There are a few reasons for this. First, even though the idea of adding flare to your wardrobe is not bad in itself, the emotional state you are in will influence how you shop. Even if you don't consciously think about it, you may be more likely to buy clothes similar to those worn by the person who got promoted over you, or to those you think will attract a particular individual to you.
Just because these styles have worked for other people, however, does not mean that they will work as well for you. If you are shopping to make yourself feel better, chances are that you have already spent several years developing a style that is uniquely yours and that suits you fairly well. Attempting, even unconsciously, to emulate someone else's style, will prevent you from expressing yourself in a style that is yours alone.
Even if you manage to avoid the pitfall of accidentally dressing like your successful neighbor, you may have a difficult time confidently wearing new clothes if you were upset when you bought them. Despite all your rationalizations, at bottom you probably know that retail therapy is an irrational behavior and that you are being frivolous with your money. This knowledge could prevent you from taking the same delight in your purchases as you would have if you had made those purchases in a secure, happy mood. If you can't wear your new clothes with confidence and pride, you might as well not have bought them at all.
So even if a new suit is the solution to all of your problems, it's better to wait until the sting of your missed promotion has worn off, and buy the suit on a good day.