Unfortunately, individuals and small businesses often try to save some money and apply for their own trademark application. Then they end up calling a trademark attorney because someone opposes registration of their trademark.
This situation may arise even if the US Trademark Office initially approves your trademark application, because your trademark is published for opposition before the US Trademark Office will register your trademark. During the opposition period, anyone can oppose registration of your trademark if they believe that they will be harmed by the US Trademark Office granting a trademark registration for your trademark.
One reason someone may oppose your trademark is if the opposer believes that your trademark is merely descriptive, or even a generic term, that anyone should be allowed to use in commerce. The opposer might argue that they will be harmed if the US Trademark Office grants your trademark registration, because they, and others would no longer be allowed to use those words to fairly describe their products or services.
Another reason someone may oppose your trademark is if the opposer believes that they have been using the trademark as their own before you started using your trademark. Therefore, the opposer may argue that they have priority and are entitled to use the trademark instead of you.
Another reason someone may oppose your trademark is if the opposer believes that your trademark is the same as, or confusingly similar to their own trademark. Someone may argue that your trademark is confusingly similar to their own trademark even if your trademark is spelled differently. For example, EXTREME is virtually indistinguishable from X-TREME.
Of course, these are merely allegations until proven otherwise. Therefore, you may be able to prevail and have your trademark registered. But it is best to work with an experienced trademark attorney who can help you navigate the opposition proceedings so that you don’t end up losing rights to your trademark for missing a deadline or making an inadequate argument.