One of the four young lovers who gets caught up in fairy magic, Demetrius is an Athenian man who's engaged to Hermia... who, for her part, doesn't want anything to do with him. In the play, he's dosed with Oberon's magic love juice and falls madly in love with Helena, whom he eventually marries.
Demetrius isn't exactly a well-developed or complex character, but we can learn a lot about the play's attitude toward love by thinking about his actions and behavior. When we meet Demetrius, he's busy insisting that Hermia should be legally forced to marry him, even though Hermia isn't in love with him. Demetrius's reasoning? He made a deal with her dad, Egeus, so it's his "right" (1.1). (How sensitive.)
Then we find out that Demetrius once romanced Helena, but ditched her so that he could get engaged to Hermia (1.1). This tells us that Demetrius, like a lot of people (and Shakespearean characters), is seriously fickle. The fact that he has already fallen in and out of love with one girl as he pursues a new love interest also tells us that Oberon's magic love juice isn't the only thing that makes lovers behave unpredictably.
Demetrius is also pretty abusive and insensitive to Helena when she refuses to give up on him. Here's how he handles her: First, he tells her flat-out that he doesn't love her and never will be able to love her (2.1). Then, he informs her that she makes him sick when he looks at her (2.1). He also insists that's she's acting like a real floozy by chasing him around (2.1). Demetrius also implies that he hopes she gets eaten by some "wild beasts" (2.1.235) and threatens to do her some "mischief" (seriously hurt her) if she doesn't scram ASAP (2.1). Then our charming boy dashes off and leaves Helena all alone in the wood, presumably to be eaten by said "wild beasts."
Demetrius's plotline reminds us that love can be cruel and that we should try to avoid falling for the Demetriuses of the world. Think of it this way: Uncle Shakespeare is basically taking you aside and saying something like "Listen up kid. This is the real world, so you don't exactly have a crew of love juice-wielding fairies to watch your back and make sure the girl/guy of your dreams returns your love."
To be fair, we could argue that Helena is acting like a total stalker. If this was the real world, we might encourage Demetrius to take out a restraining order. In the play, though, Demetrius's behavior is bad enough to enrage Oberon, who makes it his personal mission to see Demetrius treat Helena with a lot more respect.