Are the heels very high or noisy when you are walking?
Are you of below average height without heels on?
Could be because they like the look of your outfit, heels and the first impressions that the combination give.
If it is guys that are always flirty or over-friendly with you whenever you are wearing heels, it could be that they have a thing for a certain type of girl that wears very high heels (shorter girls, specific hairstyle/hair colour or very high heels).