What effect does taking antibiotics unnecessarily have on future bacterial infections?
Every time you expose a population of bacteria to an antibiotic, you select for organisms with resistance-granting genes.

Bacteria can pass plasmids full of genes back and forth.  So if you get a nasty E.coli infection, do you really want one of your usual bacteria with a resistance-granting gene sitting right next to it? 

Let's say you are so sick you go to the hospital.  Do you think the hospital wants your antibiotic-resistant E-coli around?