Romeo and Juliet experience the type of young romantic love that many of us experience in adolescence. The relationship is based on physical attraction and somewhat unrealistic ideals of love. Notice that both of the young lovers have recently left relationships where they also claimed to have been in love. The feelings they experience are real and powerful, but they are unlikely to last in this way for long. That is part of the tragedy of the play. It explores the folly of passion and romance while also showing its beauty.
Love is very different in "Macbeth." Most of the characters are part of marriages. An interesting thing about Macbeth and Lady Macbeth is that unlike most married couples in Shakespeare there is clearly still passion in their relationship. This is a stronger, more companionate form of love--the kind of love that lasts. The similarity is that Shakespeare shows how this love can be equally dangerous. Lady Macbeth continually pressures her husband to commit the crimes that lead him to becoming king and destroying his own life. In this sense, even though they have more of an adult relationship they are no more capable of escaping tragedy.
Just because love is beautiful does not mean it is not also destructive. This is a theme in many of Shakespeare's plays.