“Tonight you're mine completely”
Here we have the root metaphor Love is Possession. Not the kind of possession you see in horror movies where an evil spirit takes over someone’s body and tries to seek revenge on an unsuspecting family who's happened to vacation in the wrong secluded cabin in the woods. No, it’s more of the possession where you buy something from a store and the transaction makes you feel like you have legitimate rights and privileges over that specific item and if someone where to try steal it, you'd be pissed.
When we possess something, such as when we hold a red rubber ball, we feel we have ownership as well as control over it. If someone where to knock the ball out of our hands we would have lost control since we can no longer manipulate it. Love is Possession operates under a similar conception. We feel that “someone is ours” when they are partially under our control, when they are responsive to our desires and sensitive to our demands. When someone is unresponsive or doesn’t care what we desire, they are “not ours”. They are like the ball bouncing down the street getting further and further away.
We also tend to recognize this possession/control dynamic is partial, in that, we want some amount of control over another person, but typically not total control. That the opening line starts with "complete possession" is a familiar sentiment when talking about love that may stretch back to ideas of purity. I want 100% of your attention/commitment/energy. If there's even 1% that's something else, the whole thing is tainted (see Tainted Love).
“You give your love so sweetly”
Here we have Love is a Gift and also Love is Food. I'm giving you "my love" just as I would give you a cup of coffee or a donut. Here you go. Take it. Typically "sweetness" is relational to proclaiming something sexually desirable such as "a sweet piece of ass" where a body part is personified as a tasty morsel, but in this instance sweetness is not a quality of the gift, but a quality of the act of giving, which is a little more complex of an idea.
“Tonight the light of love is in your eyes”
This is similar to “I lost my light” in Tainted Love. The "light of love" in your eyes could mean that you are happy internally and the brightness inside is emanating from you eyes. The brightness, since it's “in your eyes” also implies some kind of knowledge since Knowing is Seeing. Do you see what I mean? If you're on a journey and you're in love then the lightness in your eyes lights up your path. You can better see where you're going and so you know what’s ahead of you and what to expect. This may be stretching it a bit far. I imagine what is mainly implied is that the person is happy or "lit up".
“Is this a lasting treasure
Or just a moment's pleasure?
Can I believe the magic of your sighs?
Will you still love me tomorrow?”
We’ve dealt with Love is an Object before, but here it's particular kind of object, a treasure, that requires a specific set of behaviors, protecting and valuing. A lover's sighs are compared to magic, which hits on the conventional metaphor Love is Magic (“We’ve lost that magic feeling,” or “the magic is dead”.) There is something unexplainable about magic. It's also typically invisible, or at least, the causal nature of the act is not perceived in the usual manner. All of these qualities can then be mapped onto love. This concept may be invoked to call attention to the unseen uncontrollable force of love that animates us and that has a degree of control over how we feel. That the line in question deals with a desire to "believe" calls attention to the perilousness of having faith in another human being, believing in their good intentions, their words, and the physiological sensations that their meaning produces in our body.
Tonight with words unspoken
You say that I'm the only one
But will my heart be broken
When the night meets the morning sun
There may be no greater cliche than "hearts breaking" but cliches are cliche for a reason, they're descriptive of a shared human experience that continues to reoccur. Typically we talk about hearts either as containers (see "Landslide" by Stevie Nicks) that have stuff in them and/or as fragile objects that need protection. I say "and/or" because you can easily imagine a coherent structure where the heart is a container that holds vulnerable material about ourselves, and because of that, needs protection from outside elements that seek to do us harm. If we are not careful, our fragile object of a heart will be broken.
The vulnerability of a heart being broken in this song is expressed in relation to the idea of being unsure of another person's level of commitment. There's an anxiety throughout that assumes that the things your lover is telling you should not be believed at face value. It's the recognition that levels of commitment fade in relation to levels of physical desire and that losing control over someone, losing the ability to exert influence over another person's behavior, could potentially be devastating to your internal conception of a meaningful emotional self, and thus, whenever navigating this particular situation, a degree of caution is advisable. Pretty good advice, if you ask me.