Now let's get one thing straight... I'm not a think happy thoughts and unicorns and rainbows will suddenly fill your day's kind of person. That's a bunch of crap.
While what we think is not going to karmically change the world... it does influence our health and wellbeing. However, being around negative people and things has a negative effect on us.
Much like a rotten apple will spread rot to adjacent apples... supremely negative people in your life will spread their negativity to you.
It can be difficult to eliminate negative people in your life as some might be in your family. Try your best to do all you can to reduce their influence on you. Avoid negative colleges, don't call back old friends who only bring doom and gloom to the table and do your best to minimise your exposure to relatives who have nothing nice to say.
It has been proven that negativity is bad for your health... now that we know that, it's time to make moves to reduce our own negativity and our exposure to that of others.
Be positive and repel the influence of the negative Nelly's who want to bring you down to their shitty dark level. There is a lot of good stuff out there in the world... choose to focus on that :)