If you have a product in mind, another platform might be more suited for the product, so it may be better to learn that.
If you have fun learning new platforms, why not go for it?
From the standpoint of generating the max amount of "code value", I'd stick to one platform. By maximizing "code value", I mean the max amount of features you can create and the cleanliness of that code given a platform. Note that this does not necessarily translate to business value or value to the consumer. Technology changes so fast, that the value of your knowledge depreciates proportional to every platform you go into. It's like for every basket you put eggs into, you lose more eggs.
In terms of happiness, you might choose a platform based on various characteristics. Do you like UI? Do you like being on the app store? Do you like the intricacies of code? Etc, etc.
For me, I like utilitarian code. It isn't flashy UI-wise or techwise, but it's clean, saves me from headaches and gets the stuff done.
In summary, if you have no goals, choose a platform and stick to it. Each platform has it's own pros and cons, it's up to you to decide what's important.