Good questions! Thanks for asking! I imagine when I start my PhD program in the Fall I'll be needing to answers to these questions concisely and often. I might as well start practicing.
Why sociology? - I am interested in examining the world's challenges from the perspective of society (how society creates problems to how society reacts and solves problems). Social theory allows us to piece together frameworks to envision critical social progress. Like Das said... it is a discipline intimately tied to social change and solutions. In addition, the most inspiring mentors I have worked with were mostly sociologists. I identified with their worldviews and ideological sensibilities so I figured it was a discipline I should take a long look at. Also, we are living in a time of EXTREME social change (i.e. globalization, internet, climate change); therefore, I believe that sociology promises a very interesting career at the least.
Why not humanities? - I got my BA in English and, by the time I was done, I was bored with critiquing literature and other texts and wanted to apply the critical thinking skills and all the theory I learned to 'real life' topics. In other words... I used to be like: "look how this theory is present in yada yada text and it is relevant because blah blah blah" / And now I'm like: "look how that theory is relevant to so and so place due to something something and here is a pile of evidence to support that inference"
Research fields: Political Sociology, Environmental Sociology, Economic Sociology, and Globalization
Attachment to the field: Curious about why humans continue to create so many of their own problems. Also professional because I want to teach, and teaching/researching in sociology is the best fit for me.