Webster's Revised Unabridged Dictionary · 1913

obsess

obsess , transitive verb

[Latin obsessus, past participle of obsidere to besiege; ob (see Ob-) + sedere to sit.]

1.
To besiege; to beset. [archaic] — Sir T. Elyot
2.
To excessively preoccupy the thoughts or feelings of; to haunt the mind persistently.

obsess , intransitive verb

To be excessively or persistently preoccupied with something; -- usually used with on or over; as, to obsess over an imagined insult.
At all ages children are driven to figure out what it takes to succeed among their peers and to give these strategies precedence over anything their parents foist on them. Weary parents know they are no match for a child's peers, and rightly obsess over the best neighborhood in which to bring their children up. — Steven Pinker (How the Mind Works, p. 449-450 [1997])