At the very opening of the play, Demetrius feels that he is in love with Hermia. In front of Duke Theseus and Egeus, Hermia's father, he begs Hermia to relent to her father's will and agree to marry him, which would thereby save her life or save her from other harsh punishment. We see Demetrius beg Hermia to give in to her father in his lines, "Relent, sweet Hermia; and, Lysander, yield / Thy crazed title to my certain right" (I.i.93-94). We further see evidence that he believes he is in love with Hermia when he asks Helena where she and Lysander have gone, as we see in the lines, "Where is Lysander and fair Hermia? / The one I'll slay, the other slayeth me" (II.i.193-194).
In contrast, although there is evidence that Demetrius was in love with Helena before he ever laid eyes on Hermia, at the beginning of the play, he feels that he hates Helena. We see evidence of his hatred in the cruel manner in which he treats Helena when she follows him into the woods in pursuit of Hermia and Lysander. He not only says hateful things to Helena, he even threatens her with physical violence, as we see in his lines, "Tempt not too much the hatred of my spirit; / For I am sick when I do look on thee" (II.i.215-126). He is even so hateful and goes so far as to threaten her with rape, as we see in his lines:
You do impeach your modesty too much
To leave the city and commit yourself
Into the hands of one that loves you not;
To trust in the opportunity of night,
With the rich worth of your virginity. (218-223)
The references to her modesty and virginity are a clear attack on her maidenhood and a clear threat of rape, showing us just how cruelly Demetrius is treating Helena, due to his hatred.
Hence, at the beginning of the play, Demetrius believes he is in love with Hermia and believes he hates Helena.
Demetrius loves Hermia because she is very beautiful. He was once in love with Helena and also made love with her (slept with her), but now, hates her because she is still in love with him and follows him around.