ob·sess 
(əb-sĕs, ŏb-)
(əb-sĕs, ŏb-)v. ob·sessed, ob·sess·ing, ob·sess·es
v. tr.
To preoccupy the mind of (someone) excessively.
v. intr.
To have the mind excessively preoccupied with a single emotion or topic: The conference organizer obsessed over the smallest of details.
[Latin obsidēre, obsess-, to beset, occupy : ob-, on; see OB- + sedēre, to sit; see sed- in the Appendix of Indo-European roots.]
ob·sessor n.