Others will just walk all over you!
If once you give them the message 'it's okay', they'll just think it's their birth right to do that every time!

Be nice to people, but be nice repeatedly - to the right people!
You really need to analyze who's worth it and who's not! Be nice to those from whom you can learn, who have something to offer back to you, not to leeches who are there just to extract from you.