In many parts of the world, growing up monoglot is quite unusual. Many people grow up learning more than one language.
It depends what you want to do with learning a language and why. Your options may be limited by opportunity - at school, we all had to do French, and to start with, that was the only MFL on offer.
As a child, I lived Roger Lancelyn Green's Tales of the Greek Heroes, and it led me to read other myths, so I knew I'd find Latin interesting, and at secondary, I had the choice of Latin or German. I knew German would be much easier to do as an adult (my mother was quite heavily involved in adult education, so I knew they did lots of things like that,) so Latin it was - right through to A-level. I did start doing German at evening classes, because I'd also told myself I would, so I did that to AS-level (as far as I could go with evenings.) Also had a German boyfriend through part of my 30s, which helped, though he was fluent in English.
Then because I could read some Spanish because of having French and Latin, I did that to AS-level in evening classes, too. I find I can understand some Italian, but as yet have not found time for learning it, and wouldn't be able to speak it. As I got to a point where I couldn't take German or Spanish further at evening classes, I signed up for Russian (have done quite a bit of Russian history in the past,) but they didn't get enough people enrolling to run it, so I ended up doing Welsh instead, which I'm still doing, around B1/B2 CEFR level, with classes now online rather than in person as they started.
I'm doing all those on Duolingo to keep my hand in, but I find it frustrating to learn from scratch there, as I'm doing with Dutch, which is mostly to prove a former colleague wrong about no one ever bothers learning it... I'm finding it fairly easy with knowing German and English, but Duolingo is rubbish at explaining grammar, so I've bought a grammar guide alongside so I know what it's trying to teach me.
I also have a tiny bit of Malay from having spent some months in Malaysia. Couldn't do more than get round a food menu these days, though. Boyfriend has Mandarin after living in China for 6 or 7 years.
I worked in IT for about 30 years, and that was partly because it was like learning another language.
I like languages, particularly the patterns and etymology and links between them. I've always known why I chose each language I've learnt. I don't speak any fluently. But the interest makes me quite a good learner. Opportunity is important - less so now we have the Internet (which we didn't in my childhood and 20s.) I know there have always been teach yourself books, and sometimes tapes/CDs, but that is nothing like being with a real, interactive teacher. Learning languages is much easier these days in terms of available resources (apart from funding for adult education, which is dire these days.)
But you need to have some reason to motivate you to learn. For many people, finding language generally interesting isn't enough. There needs to be travel, work, love or something to be a motivation. It takes time, which can be challenging when you have a job to bring in money, a house to run, etc. Had i more time, I've a list of other languages I'd learn...
Opportunity is key - not everything is on Duolingo etc, there may not be evening classes or tutors (real or online) available. A school can't offer a language thry don't have a teacher in - and just knowing a language doesn't mean you can teach it. It helps to have real speakers to talk to for practice, which may not be possible.
But I think choosing a language because it might be useful, and then never having an opportunity to use it isn't likely to result in much learning unless you are the sort to enjoy language learning for its own sake.