This myTake was made for straight guys but can apply to women and the LGBT community too who approach people. Here are some reasons why approaching has not worked for some people.
1. Sneaking up behind them or approaching them in a creepy predator like way
OK you can't be mad if you scared them away before you could say hi when you took the jack the ripper approach to it. Also if they are by a pool or sitting on the train alone at night breathing behind their neck is not the best idea.
2. You pass them a note with your number on it, but you say nothing at all and you make no effort to introduce yourself
Seriously why should a person put in the effort to call you when you don't even say anything but give them your number and walk away?
3. Approaching them while they are on the phone
4. Too aggressive and pushy
Assertiveness is fine, but when it's obvious the person has no interest and she tells you, you should back off. Some people approach with this sense of entitlement and they do not realize that it comes across as scary, intimidating, or too aggressive. Even if you meant well, it's best to move on because there are plenty of fish in the sea.