Great question. It's one of the things that you can either see as Shakespeare being a bit lazy, or as one of the things that really complicates the play.
Firstly, Demetrius does not love Helena at the start of the play. Shakespeare has Lysander tell us that he's slept with her, but then decided to marry Hermia:
Demetrius, I'll avouch it to his head,
Made love to Nedar's daughter, Helena,
And won her soul; and she, sweet lady, dotes,
Devoutly dotes, dotes in idolatry,
Upon this spotted and inconstant man.
Demetrius then pursues Hermia to the forest. And when Puck eventually gets the potion on his eyes, he sees Helena, and responds with very, very, very passionate language. It doesn't sound like an illusory love:
O Helen, goddess, nymph, perfect, divine!
To what, my love, shall I compare thine eyne?
Crystal is muddy...
But then again, he never felt it before. And when he wakes up again at the end (still under the potion's spell) he tells Theseus
...the object and the pleasure of mine eye,
Is only Helena. To her, my lord,
Was I betroth'd ere I saw Hermia.
But, like a sickness, did I loathe this food;
But, as in health, come to my natural taste...
Is it "health" - is it his "natural taste"? Or is it just the potion talking? The answer is, there's no way to tell, and it's up to you to come up with your interpretation. For me, I've always found Demetrius' magical-love at the end of the play really quite disturbing! I want to know what he really feels!
I must disagree with robertwilliam's assessment of Demetrius' feelings for Helena. "Made love" in this context did not necessarily mean what it means to us today; more likely it means that he courted Helena, which makes more sense with the next line "and won her soul." Demetrius and Helena were in a relationship prior to the events of the play, but he broke up with her. This does not mean that he never loved her; in fact, evidence supports that he did.
...To her, my lord, / Was I betroth'd ere I saw Hermia.
For ere Demetrius look'd on Hermia's eyne, / He hail'd down oaths that he was only mine...
There is also evidence from Demetrius himself that they have not slept together:
To trust the opportunity of night/ And the ill counsel of a desert place / With the rich worth of your virginity.
As for westmontgirlshc's question, it's really up to the reader to decide! I like to think that by morning the love potion has worn off, but the experience of it has opened his eyes and made him realize his mistake - he loves Helena, and he always has.
it is the illusion of love because if you take the example of titania and bottom she had never met bottom before its just simply because she was under the spell. demetrius' love for helena previously just makes his love stronger whilst under the spell.