To preoccupy the mind of excessively.(verb-transitive)
To have the mind excessively preoccupied with a single emotion or topic: "She's dead. And you're still obsessing” ( Scott Turow). (verb-intransitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "obsess" in a sentence
"Ah, hell, I obsess from the moment something is completed."
"The word obsess showed up in English straight out of Classical Latin back in 1440."
"So, can you now get another issue to "obsess" about?"