If, as Christians (must) believe, Love is the most important thing there is - then of course Love cannot actually be defined (because that would be to define the more fundamental in terms of the less fundamental) but what can be said about it?
One distinction is to ask what kind of a thing Love is.
To which several answers have been given.
Modern culture regards love as a psychological state - but clearly that won't do; because psychological states are evanescent, and constantly changing - and have such individual applicability that they cannot convincingly be extrapolated to be the most important thing in the universe.
To regard Love as a psychological state inevitably trivializes Love; and is therefore intrinsically anti-Christian.
(...Which is why Love as a Psychological State has become so popular in secular modern culture - and why this belief has now been made mandatory, and why this belief is now imposed by coercive force. Modernity is Leftist, and Leftism is built-upon anti-Christianity.)
Classical medieval theology regards Love as something physics-like - something which (for example) keeps the stars and planets in their orbits.
This is much better, but such a way of speaking (such a metaphor) strongly tends to make Love into some kind of impersonal force - something much like gravity or magnetism.
And thereby this renders God into an impersonal force. By regarding the universe as created and sustained, as held-together by Love, Love (if it was really real) becomes something we would expect to detect and measures with sufficiently sensitive instruments. And when this doesn't happen, we may lose faith in love.
Although this kind of abstract conception of Love would be appropriate for impersonal religions such as Hinduism and Buddhism, an abstract metaphor is the opposite of what is needed for Christians where God is our Father and Jesus is our Saviour.
My suggestion is that we regard Love as being about families - as a thing which happens primarily in families; ; and families as being organized-around the process of generation.
This means that the paradigm Christian Love is the love of husband and wife (where the marriage relationship is a sanctified sexuality); and the love of parents for their children; and the love between siblings; and the love between the whole network of such relations - the extended family, the clan, a 'people'.
All these are properly considered as different inflections of the same basic kind of thing.
The love is about 'generation' in the double sense that it is about (organized-around) the procreation of new generations and that it is about (organized-around) what binds different generations.
In divine terms, the emphasis is that all people have at least a parent-child relationship with God, and therefore sibling relationships with one another.
(And as well as this minimum Love, some people are united by different forms of more-than-this love - husbands and wives, brothers in families and so on.)
But this universal minimal Love of God for Father and all Men as siblings leads to the 'adoptive' aspect of Christian love (sometimes emphasized by St Paul)
...Which I would interpret as implying, on the one hand, that the fullness of Love is necessarily a voluntary choice. We just are God's children and united by brotherly love, like it or not; but this real relationship may be acknowledged and embraced; or (perhaps dishonestly or ignorantly) denied and rejected: that is a choice.
And on the other hand, that the revelation of the sibling relatedness of all men means that there is a mystical 'transferability' between earthly families. In other words, earthly 'genetic' relatedness - such as the genetic links of a family, clan or a people - is ultimately superseded by a larger Heavenly concept of Christian sib-ship - universal Brotherhood and Sisterhood.
Which simply means that family is NOT reducible to genetics; and generation is NOT reducible to reproduction - they are of course about this things, but there is (vitally) much more to the concept of family and generation than merely 'science'.
What would this idea of Love as being about families and relational mean in terms of the physics of the universe?
The answer would depend on the basic metaphysical understanding.
One answer could be that Love 'works' to bind and run the universe in the sense that everything (everything) is alive (more-or-less alive); and that there is an affiliation between the living.
So what we call gravity would be, in transcendental terms, something like an extremely weak and impersonal form of love.
Another answer could be that at least some of the 'physics' of the universe (basic matter, and what we recognize as the 'laws' of physics - what things are and how things interact) may be a 'given' - that is not a part of God's creation but something God works-within.
For example, it seems to me that Time just is linear and sequential - it can go faster or slower (e.g. along the lines described by Einstein, but not necessarily constrained by Einstein's discoveries); but there is nowhere outside of Time, and Time cannot be reversed - nor is Time travel possible (and any physics theory which states that such things can happen is ultimately wrong; no matter if such theories have some pragmatic value).
Thus Time is a constraint for God as for us, it is a 'given', something we live 'within' and within-which Love and Families and Generation happens.
And therefore Time (and by extension 'matter' and 'the laws of physics' are not a part of the economy of Love; but are the backdrop to the drama of Family Love which is the meaning and purpose of the Universe.
But however we fit families into all of reality (or all or reality into families) - families provide the vocabulary, the metaphor, the best and truest way of talking about Christian Love.