The way you word your question is very suggestive. Investing in bitcoin is a choice, you can either do it or not. There are plenty of other ways you can earn money, safer alternatives than bitcoin.
Currently bitcoin is purely a speculative currency, there's nothing backing it, it's massively slow due to blockchain limitations, and therefore cannot be adopted by mass market. But you can think about investing in it because it can yield you profits, profits that would take you years to make using traditional investment instruments. This to me is the only reason why you should invest in bitcoin.
Why should you invest in Ethereum, however is a story for another day.