Students of linguistics have probably heard of the Sapir-Whorf Hypothesis. It essentially states that the languages you know directly influence your understanding of the world. In other words, the languages that you know influence how you think about and approach problems.
Though often applied to natural languages, this can certainly be applied to programming languages as well. If we have had exposure to multiple programming language paradigms, such as declarative, functional, and imperative, we will be better at solving problems by choosing the right tool for the job.
Certain language paradigms work better for some certain classes of problems. For example, query languages such as SQL work well as declarative languages because we're more interested in what to find rather than the exact steps of finding it. Declarative languages are also a great way to specify the view for a program. We see this with XML for Android and XAML for Windows WPF applications.
Then we have functional languages, which extol the virtues of pure functions that are free from state and side effects. This makes it easier to reason about and test our programs. It also allows us to take advantage of lazy evaluation and to parallelize our programs quite trivially.
Finally, imperative languages allow us to specify how to accomplish a given task. This is important when performance is a concern or when we need more granularity.
Fortunately for us, many of the newer languages are multi-paradigm, allowing us to use declarative, functional, and imperative ideas in a single language.
So learn a new language, preferably of a paradigm that you're not as familiar with. Even if you rarely utilize the language itself, the concepts gleaned from that language will make you a better programmer and problem-solver.