If you find someone attractive, it's natural to want to be nice to them. It's also natural to want them to be attracted to you, too. Unfortunately, the combination of these two desires can lead people down some very dark and destructive paths, paths that end in loneliness, hurt, anger, and the total destruction of any chances of a meaningful romantic relationship.
Two of the major missteps are being overly nice and being manipulative, though they are deeply connected.
By being “overly nice,” I mean having an attitude of servitude. This is both demeaning and no basis for a relationship.
To behave as a slave toward someone, catering to every whim, just diminishes your personality. Relationships thrive in the mixture of personalities, so not only does behaving this way demean you, it also precludes a genuine relationship. Moreover, it leaves you open to the other person taking advantage of you.
Needless to say, being a simpering fool is unattractive, probably because humans innately understand that such people do not make for good partners.
The second misstep is having the motivation of trying to control or influence reciprocal feelings. If you pretend to be better or behave in unrealistic ways in order to impress someone, you're lying in an attempt to manipulate him or her into liking you. Even if you change in objectively positive ways; you're still building your character around infatuation, which is a perilous foundation.
Tuning yourself around your latest crush is not a realistic long-term strategy: You must be able to stand on your own and improve yourself on your own to be part of a relationship, because a relationship involves supporting as surely as it involves being supported.
These two missteps are related. The first attitude has some elements of the second. By being a slave, you're trying to influence the other person to like you through “niceness.” You may even be emulating some aspects of virtue, since loving service can be virtuous. But diminishing your character is a lie and therefore manipulation.
Additionally, making oneself a slave and having the motivation of making a particular person attracted to yourself both tend to objectify people. Being a slave and catering to another's every whim without complaint implies an attitude that the other person would want a slave as a partner; in other words, you invite the object of your attraction to objectify you.
Trying to manipulate someone into loving you by lying about yourself, whether by being his or her slave or by other means, does not treat the object of one's affections as a person to be related to but as something under one's control: not a person at all, but more like some sort of toy or plaything.
To avoid these mistakes, treat the other person as a human being. Accept that you're attracted, and test the waters - try to get hints as to whether he or she's attracted too. If not, it may hurt, but it's best to accept it: the alternative is to make mistakes, such as the ones above, which will even in the best case just be a waste of your time. Spend time with him or her, try to understand who she or he really is as a person.
In the end, and this is really hard to accept, one person having feelings for another is not romantic love. Romantic love only really happens when both people have feelings for each other, and it's not something you can control or should try to control. If you try to control how the other person feels, even if you are “successful” you will have failed. Failed in many ways, but most of all, failed to really love the other person.