One of the fruits of the Spirit (Gal. 5:22-23) is self-control, and it applies not only to what we do but also to what we think. For example, not being anxious (Matt. 6:25) and thinking about good things (Phil. 4:8) are instructions in the Bible and those both require self-control in thinking.
A book I read once claimed that too much TV watching was the reason for less self-control of the mind in many today, saying it is a very passive activity. I don't agree with that because you can watch TV with your mind fully awake and critically thinking about all that you see.
It's true that what we see on TV might encourage not having self-control of the mind, since it reflects our current culture and our current culture is postmodern, which believes that truth is relative and therefore the self is where truth is found. But TV watching is like talking to a person. You don't have to agree on everything to have a meaningful or fun conversation, and talking to someone with a different viewpoint doesn't mean you'll soon adopt that person's viewpoint.
One way to help with developing a more self-controlled mind is to remind yourself often of truths in the Bible, and to try to be self aware of how often you are or are not authentically thinking positive thoughts. We are often surrounded by negative people, so much so that it's hard not to fall into that habit of negativity and possibly not even really notice how negative we've become.
There's never a day when there isn't something someone could complain about if they were looking for something unpleasant in their life. For example, waking up in the morning is never easy, and there could always be more coffee or more of something else we find we'd like to have. But it's draining when we or when others start to complain. And the Bible says we should try to help others and aim to be healthy. There's a place for sadness and it's good to see truthfully a situation, even when it's a very difficult one, but there's a difference between seeing reality and unnecessarily pointing out the difficult realities of life.
People may wonder how to be positive when everything seems so terrible in their life. I'd say that little by little it's possible to truly understand what a gift even life is. God has given us life. Every breath. And the possibilities that life brings. There's also beauty in this world. Laughter. Hope. Jesus's work for us, when we are not deserving in the least, but he loves us anyway.
There is so much to rejoice in when it comes to who God is and what he has done for us. Even the stories of his work throughout history is something to be amazed at.
One reason self-control of the mind is so important is that our minds have limits. Our minds can get tired too. If we spend so much time on negative thoughts, there's not much mental energy left to truly think on positive things.
And of course the most important reason is that we should do everything, including thinking, to the glory of God.