Being "nice," just like any other behavior, human or otherwise, is a means to an end. It is helpful so long as it is beneficial, but as the old saying goes, too much of anything is a bad thing. When it becomes more harmful than helpful in any way, it has gone from beneficial behavior to harmful tendency, and is therefore very much a handicap, since it disables you rather than the originally intended purpose, which is the enabling of something by gaining the positive feelings from another individual or group.
Being too nice is a handicap. There is a time and a place for
everything. I believe that we should
treat people the way that we want to be treated, but only if they treat us the
way they want to be treated. Ignorant
people take kindness for weakness, so we have to be careful.
Nice can be good, but if it falls into the realm of people pleasing, it can turn into a character defect. It hurts the person him or herself, but it can also hurt others. That is because someone who is too nice and a people pleaser does not make decisions with integrity but rather based on what they think others want, so one can't be sure of their conviction.
While it's always nice to be nice it's never nice to be a pushover. Many people today think any form of showing assertiveness or saying no is rude and so they become pushovers in order to avoid conflict. We need to have a global cultural change, fast, because that attitude is just stupid. But you can be nice, while still maintaining your agency and integrity. Just smile and say no!