Some years back, a memo from Roddenberry came out that clarified his intentions for Spock in a disturbing way. I don't have a link to it -- hopefully someone here will remember where to find it -- but basically, Roddenberry's rather creepy idea was that Vulcan males had a Svengali-like hypnotic effect on women, necessary to induce passion in their "frigid" Vulcan mates (ick). So Spock couldn't help having human women fall desperately in love with him, though he struggled not to take advantage of this power. That's why Chapel and Leila were so obsessively in love with him and why he couldn't let himself respond. (The one time we did see him using his hypnotic woman-enslaving power was in "The Omega Glory," since that was a Roddenberry script. But it came off as just a variation of his normal telepathic powers, since we weren't given the intended context, thank goodness.)