BrightEye,
Here's a different perspective that I haven't seen mentioned. As part of my deck study process, I've laid the Majors out in 9 columns. When done this way, the Lovers and The Devil are in the same column because both cards relate to the number 6. Numerologically, 6 can mean many things, but here's what I just wrote about the relationship between these two cards (while in the back of my mind remembering there was a thread that discussed these exact two cards):
"Sixes are cards of choice, harmony and taking responsibility. The Lovers is about making choices and accepting the responsibility of choices that have been made. The Devil suggests the need for one to choose to take responsibility for their addictions and address them in order to break free of them."
In thinking further about it, the Lovers seems to be about making the choice to move toward something while the Devil seems to be about making the choice to move away from something.
So with regard to the original question, I think that the Lovers card as someone's feelings for another would be a positive card since it would suggest that this someone is making the choice to move toward the other person with the intent to form a relationship with them. (Of course, I'm assuming that there are other cards and that they reinforce this particular interpretation.)
BrightEye said:
Or does the devil have a negative side always attached to it....
This question kind of got glossed over and never really answered. All of the cards have both positive and negative meanings to them. The Devil is usually seen as a negative card, but it doesn't always have to be negative.
If A were only interested in B for sex and consulted the tarot about B's feelings for A and got the Devil, the Devil would probably then be a good card since the suggestion would be that B is sexually interested in A and we already know that A is sexually interested in B.
A friend who reads was doing a spread about her future love life and asked what type of man she should be looking for. She got the Devil and was really confused by that card. So she asked a bunch of us what we thought and the consensus was that she should be looking for a physical relationship with a man who is sexy and/or sexual.
If I got the Devil as an answer to "Where should I be focusing my energies right now?" I would take that to mean that I should be trying to free myself of something (or someone) that's holding me back.
I take all three of those examples to be positive sides of the Devil card. Now if A were in love with B and asked about B's feelings for A and got the Devil, that wouldn't be a good card for A.
Hopefully something I've said has been of use, even if only to kick start some other thought.
Rodney