The Lovers is a 'thinking man's card'. It throws the net so wide that it pretty much encompasses any issue you can think of regarding romantic relationships. I think The Big Three are duality, reality and perception, and I'll often make mention of all of them when this card shows up in a reading.
First thing's first: duality. How much do you have in common with the other person? Do common interests actually matter that much? Has the lack of a common interest or outlook created a misunderstanding between you? As we walk along this crazy yellow brick road of life, priorities change and we select our companions -romantic or otherwise- to mirror our natures, challenge us and aid us in times of struggle. Our needs don't stay the same throughout life and trying to force old ideas into a new stage of existence is only going to create rigidity and unhappiness. Can you both evolve together and show understanding for personal changes you've gone through? If not, is that as it should be?
Reality is a big chunk of the card's message and rightfully so. The recipe for long-term love is made up of more than one ingredient. Magical, storybook romance isn't the whole animal and the card encourages all who receive it to come down to earth when it comes to romantic attachment. How much do you invest into the reality of your relationship? How many hopes and expectations are shattered and is this a sign that your ideal scenarios are actually unrealistic? Do you put as much energy into the everyday grind of life with your partner as you do into the overblown gestures, romantic getaways and sentimental feelings? Do you find that your partner is a realist in general and that this balances out your flighty or irresponsible nature? Can you help your partner with their practical concerns and problems? (All of these questions can be reversed to approach the issues from the perspective of the other partner.)
Last but not least, perception. Every card has a warning element. (Yes, every card, even the most overtly positive.) The Lovers warns of seeing hurdles in the relationship as barriers rather than challenges. Barriers are off-putting, depressing, worrying and sometimes impossible to overcome. Challenges are energising, exciting and encourage personal growth and experiential learning. Are you getting the two confused and therein causing yourself to become pessimistic, negative or despondent? How can you work towards changing that? Can you be honest with your partner about your fears? What can be done on a practical level to change your perception and do you need to listen more or less to the perspectives of others?
Some readers like to branch out with The Lovers and state that it also encompasses our love of our passions and life goals. I don't see it that way. For one thing, there's an entire suit in the Tarot for addressing those particular concerns (namely Wands). But more importantly, in every deck I've ever used, the symbolism in The Lovers card always denotes a romantic connection and that's where I think the essence of the card lies. If the card shows up in spreads that have nothing to do with romance or for a querent who is single, the emphasis could be on future hopes, past romantic issues or the wider effect of someone else's romance. I always give the card the chance to be what it is and read it in its romantic context. Too much manipulation of the card's meaning can result in missing the point of the message entirely.