If you have a good reason to learn a language and are highly motivated, you’re more likely to succeed in your efforts than those who only have a vague idea why they want to learn a language and go about it in a half-hearted way.
If you regularly come into contact with people who speak a foreign language and who don’t speak your language, or who speak your language but feel more comfortable speaking their language, you’ll have a very good reason to learn the foreign language. You might find yourself in this situation if you live in a foreign country, if you visit foreign countries regularly, or if your company has many foreign clients and/or customers.
Another good reason to learn a language is to understand the conversations of others, especially if you suspect they’re talking about you. This can be useful when you’re abroad, and also when you encounter speakers of other languages in your own country.
There’s an urban myth that when English-speaking people go into pubs in Wales the locals will start speaking Welsh so the new arrivals cannot understand. This assumes that the locals were speaking in English beforehand, but how would the English speakers know that? Indeed, how could be sure that the locals were speaking Welsh if they don’t speak it themselves? Having lived in Wales for a year I’ve never observed this happening, but I’ve heard of one case in which a number of Polish people were talking in Polish in a pub and the person who went in assumed it was Welsh. Welsh speakers tend to speak Welsh among themselves if all present speak it, but it there’s an English speaker in the group, most of them, if not all, usually switch to English.
People often assume that if they speak in a foreign language nobody around them will understand, and so it’s safe to make comments about people. This is a dangerous assumption to make as you never know when you might come across someone who understands your language. In fact if you regularly encounter someone who speaks a foreign language and appears to be talking about you and others around you, you have a good motivation to learn that language. There’s an example of this in this video, in which an Irish psychologist’s receptionist makes scathing remarks about the patients while talking in Irish to a friend on the phone. None of the patients know Irish when they start going to the psychologist, but they are spurred to learn it so they can understand the receptionist.