Raga means attachment, liking or desire for something and dvesa means aversion, dislike or hatred for something. So the relation between the two is that they are opposite to each other. But because the world is filled with dualities like hot-cold, pleasure-pain, etc., liking for something can automatically create dislike for the opposite of that thing. For example, if one likes hot coffee and tasty food then he will dislike cold coffee and tasteless food. If one has a liking for winter season, then he may dislike the others. Similarly, if one dislikes the winter season, then he will like any other.
So whether it is raga or dvesa, one can lead to another because the world is filled with objects of both kinds called as dvanda in the scriptures.. So Shri Krishna instructs to get rid of both of them. Whatever may it be, raga or dvesa, when one has it for certain person or certain being, he is bound to think about it all the time. If a person likes someone then he will think about him, and if a person hates someone, then also the person will think about him. So either of them is bad. Both has to be get rid of to fix the mind on God. Generally spiritual practitioners develop attachment towards God but in the way they tend to dislike samsara or the material world. But that is wrong, there should not be liking or disliking for anything in the world.
In that verse (BG 3.34) Shri Krishna tells that, this raga and dvesa exist in the senses because through the senses that the objects of the world is felt. And due to the nature of the world, while certain things are pleasurebale to the senses certain things are not. Due to the three fold material nature every one has liking and disliking for certain things as per their own nature. Every body is born into this delusion of dualities of the world:
iccha-dvesha-samutthena dvandva-mohena bharata
sarva-bhutani sammoham sarge yanti parantapa [BG - 7.27]
O scion of Bharata, O conqueror of the foe, all living entities are born into delusion, bewildered by dualities arisen from desire and hate.
But because either of it causes attachment to material things and not God, Shri Krishna instructs to get rid of both.