It is soooo easy to allow negativity in--wrong thoughts, harsh criticisms, unnecessary arguments, unpleasant conversations etc. Not only does it not benefit us in anyway, but it also leaves us feeling uncomfortable and bothered. And like attracts like, so by allowing what seems like a "just-this-one-time" bit of negativity in, we attract even more negativity to ourselves. And you can keep going with that thought on how negative thoughts affect not only our mind frame but also our lives ultimately.
I started reading "The Secret" recently (yes, I know, I'm 10 years late). There has been so much controversy over it but like I was telling a friend of mine on the phone today, you take what you need from things. And what I got was a new found consciousness to guard my space ("Take Care of Your Space" blog post linked here) even more and allow only the good thoughts in. And every time I feel uncomfortable or bothered by something, I do a quick scan and immediately switch to a good thought.
"Let the sun shine in
Open up your heart and let it shine on in
When you are lonely, let it shine on
Got to open up your heart and let it shine on in
And when you feel like you've been mistreated
And your friends turn away
Just open your heart, and shine it on in"
The Fifth Dimension