This is a great question and immensely provocative.  So what makes me think? Do I do it simply because I am conscious?  It appears so since consciousness and thought seem constantly connected.  But apparently I also think when I am unconscious as well such as when I dream.  So what makes me think?  I would say simply existing causes me to think.  So jbow, with your indulgence and permission I'm going to expand the question a little and ask "What makes me think deeply".   I humbly offer the following:
 
questions such as "What makes you think?"
mystery
teilhard de chardin
every philosopher that has ever lived
human behavior
The Enneads
faith
love
The One
 
Thank you for making me think!!
 

I get a bit freaked out when I start thinking about why, out of all the people on the planet I could have been, I ended up being me.
 
Then I have a cup of tea and forget about it.

 
Images like the above.  When you first read that it's taken by the Hubble telescope aimed at a tiny point in the sky attempting to look as far away as possible, that's mind blowing enough.  But, when you realize that almost every bit of light that you see is actually an entire galaxy similar (or more likely larger than) our own Milky Way, well...
 
 

What mak....... oh squirrel!!!!

 
Squirrel???  Where??!

craigb

 
Images like the above.  When you first read that it's taken by the Hubble telescope aimed at a tiny point in the sky attempting to look as far away as possible, that's mind blowing enough.  But, when you realize that almost every bit of light that you see is actually an entire galaxy similar (or more likely larger than) our own Milky Way, well...
 
 

...all that, and we let this happen...


https://www.youtube.com/watch?v=9G4jnaznUoQ
 

Life's mysteries is what makes me think.

Electro-chemical processes.

57Gregy
Electro-chemical processes.

Yeah, right.  As if!