Hey there! I'm happy to help!
The subjunctive form in Spanish can be used for lots of different things, but in this case it is used to persuade or influence. Let's translate these two sentences to see which one seems to persuade or influence.
1. I want you to arrive on time.
2. You never arrive on time.
Two is just stating something, but it isn't encouraging the person or giving a command. The first one is trying to influence the person to arrive on time, so sentence one uses the subjunctive here.
The subjunctive tense also has a different form. We are using the verb venir. Vienes is the normal present tense for tú, while vengas is the subjunctive one (has an "a" ending instead of "e" or "i".
Have a wonderful day! :D