Question: "Why do we have to dress up nice for church?"
Answer: It is good for us to ponder why we wear what we do. Genesis 35:1-3 may shed some light on the subject. "Then God said to Jacob, 'Go up to Bethel and settle there. Build an altar there to honor Me. That's where I appeared to you when you were running away from your brother Esau.' So Jacob spoke to his family and to everyone who was with him. He said, 'Get rid of the strange gods you have with you. Make yourselves pure, and change your clothes. Come, let's go up to Bethel. There I'll build an altar to honor God. He answered me when I was in trouble. He's been with me everywhere I've gone.'"
It is possible that as Jacob began this faith-journey to Bethel with God, he recognized how much God had done for him, and how much he needed God! His response was to take everyone with him on this faith-journey, so they could experience God for themselves. "Get rid of the strange gods you have with you. Make yourselves pure" implies the need to be united in "coming clean" before God. "For all have sinned..." (Romans 3:23). Many then had "household idols" with them that they depended on, as well as God. They did not trust God alone. "Change your clothes" implies a change of heart toward sin. It was to be a reflection of what had taken place on the "inside."
We would all benefit from a "spiritual bath" to confess and get rid of sin before we go to church. This is making ourselves pure. For some people, clean is their "best." For others, their heart tells them that wearing their best is showing God His value to them. For still others, there needs to be a caution that their best isn't merely showing off.
It is always the heart God is looking at, rather than the exterior. However, what we wear to worship our holy, pure God may be an indication of where our hearts are. If you have never considered it before, ask yourself, "Does it matter to me how I look when I am going to worship the King of Kings and Lord of Lords? More importantly, does it matter to Him?" We must all be the judge of that for ourselves. It's a personal choice, keeping in mind that having a proper attitude toward God Himself is important preparation for worship at church.