I need to work out some issues with the opposite sex before I even consider a relationship.
Other times, I just know they're not the type for me, especially if I feel they're 'out of my league', too good for me, etc.
Example:
You're outgoing. I'm mostly aloof. You constantly talk of getting high, drunk, etc. on a weekly basis. I'll drink, but not nearly enough at the rate to destroy my liver like you are. Plus, I not attracted to women who smoke. I like a routine. You're very spontaneous. You love getting attention. I do not, plus I might not be able to give you attention like you want me to, or I might give you WAY too much of it. You're impatient, and you can't be dealing with me. You seem to be arrogant, a practical narcissist, when it comes to how attractive you think you are, and your ability to attract men. I don't like people like that. That's why we'll never work.
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There's no Heavier Burden than a Great Potential! - Linus