A good boyfriend cares about me. If I'm sad, he's there. If he's sad, he lets me be there for him. He understands that I am not perfect and we will disagree, but that doesn't mean me can't be together. He accepts my flaws, and lets me love his. A good boyfriend is the best-friend that you share your whole self with.
Also a little bit less ambiguous: Buy her flowers randomly, say how she looks so pretty, notice the little crinkle in her eyes when she smiles and comment on it. The little things that you love, say them to her. Let her know that you appreciate even the little things. When you fight, listen. If you are wrong say sorry. Don't give up just because you fought, fighting is inevitable. But try to move forward from that and work to be better. Being a good boyfriend is hard, as is being a good girlfriend.
The term good boyfriend can be seen as subjective, people will have their own definitions of what is seen as a " GOOD" boyfriend. Yes, I do have the definition of what would make an excellent boyfriend. I'll tell you what makes an ideal boyfriend according to me and then you can pick up whatever you think is right.
1. An ideal boyfriend will always be understanding, loving, caring supportive and towards his girlfriend.
2. An Ideal boyfriend will never or almost never fight with his girlfriend, he may have his views and opinions and will always express them in a very professional way. He might be assertive but will never be aggressive in expressing himself.
3. An ideal boyfriend will always be logical while taking decisions ( about himself alone) however he will also care and respect his girlfriend's emotions and feelings if she is a person based on emotions/feelings.
If you understood my point.
4. An ideal boyfriend will never be judgmental, rude towards his girlfriend.
5. An Ideal boyfriend will be a very mature person, he would be a true man, a man of character with lots of self respect.
6. From point #5 An ideal boyfriend will never ever try to make his girlfriend jealous using another woman in any way you can imagine. He will never play such stupid, immature mind games to test his girlfriend. Hence he will never manipulate her in any way or even think about doing that.
7. An ideal boyfriend will never play hard to get. He will never make excuses, he will never keep his girlfriend waiting for replies, texts and so on. His attitude, thinking would be such that he will reply to her as soon as possible. Hence he will be a person with excellent communication skills. In case of delays he will always apologize and give reasons and updates as to why they delay happened.
8. An ideal boyfriend will never look at other women, will never have a wandering eye. His thinking would be such that he will have eyes only for his girlfriend.
9. An ideal boyfriend's attitude and thinking would be such that he would be totally immune to cheating, even the very thought of cheating won't ever enter his head. The reason being he would be a very logical person and so he won't be greedy. He will always be content and satisfied with what he has and according to him that would be the best.