There's nothing wrong with being happy. The pursuit of happiness can even be a godly activity. But to know happiness in its fullness, we need to keep God in the equation.
Look at it this way: When the Good Samaritan helped the injured man by the road, God certainly evaluated his work as good and upright, and the Samaritan also reflected God. But chances are, the Samaritan felt good about what he had done as well. He likely experienced pleasure that he had really helped someone in need and had pleased God. He may have been inconvenienced by the help he gave, but that doesn't mean he was being self-sacrificial. He loved his neighbor as he loved himself, and self-love is part of happiness. Thus, the Samaritan was happy in all three senses that Wesley identified.
Here's something else: Almost certainly the Levite and priest who passed by the injured man without helping didn't arrive at their destination as happy men. They had no doubt come up with some sort of justification for their decision to pass by on the other side, but such justifications don't yield self-love. They support selfishness (which is different from self-love), but they don't result in a feeling of pleasure and satisfaction.
As we learn to enact the kind of self-love the Good Samaritan showed, our happiness deepens. And, in effect, we are rejoicing in the Lord, as Paul recommends in today's text. When Paul told the Philippians to "Rejoice in the Lord always," he wasn't recommending a worshipful ritual, but urging his readers to feel the genuine delight that comes from living and acting God's way.
Happiness, the Bible teaches us, is a feeling that comes from doing what pleases God.