"It's OK that he/she does that....I can change them."
"I don't know why you don't like him...people can change, you know."
"It's just something that she does now. Give her time and she'll quit. I just have to change her a little."
If these questions are ones that you have contemplated concerning your current relationship, stop. You cannot change someone. This is a realization that takes humility to admit. It's hard to think that, despite your love for him/her, you can't change their ways.
This is not to say that people cannot change. This is as far from the truth as possible. Look at the amazing stories of recovering drug addicts and reformed criminals. Indeed, people change. Yet it is not you that change them - it is God.
In all actuality, how can we change someone when we struggle to change ourselves? How many of us have made New Year's resolutions only to break them on day two? Or, as Lent approaches, have a list a mile long that we want to improve ourselves with?
When in a relationship, realize that the person you are interested in is already having a path of trajectory. In a relationship, you join in the effort to work towards a common lifestyle together. If the boy you like is on a trajectory downwards, when you join him, don't think you can turn that path in a complete 180 turn. More than likely, you will be joined in the trajectory as you become more and more involved with him. You will then find that in your effort to change him, you have in fact changed yourself.
You should be in a relationship where both of you are working on a goal: to get each other to Heaven. This can be an incredibly hard challenge if there are obstacles standing in the way of that goal. Do not be caught in a trap that leaves you thinking that you can move these obstacles alone.
God bless you all,