In Act 2, sc. 1, Helena tells Demetrius that he can treat her like a spaniel (dog), "I am your spaniel,..".  Further she says he can spurn her or hit her or just generally treat her very badly.  She tells him he can do this because she simply wants to be near him.  She is so much in love with him that even if he treats her like dirt, as long as she is near him, she will be happy.  Helena obviously has self-esteem issues because she debases herself.  Demetrius loves Hermia and wants nothing to do with Helena despite the fact that they had a previous relationship.  The traditional roles are reversed:  Helena, the female, is persuing Demetrius, the male.  Helena even remarks on this: "Your wrongs do set a scandal on my sex. / We cannot fight for love as men may do. / We should be wooed and were not made to woo."  She is venting her frustration on this role reversal and how she isn't good at it.