Why I am Not a universalist with respect to salvation

The basic reason is that I have an extremely different understanding of ultimate, metaphysical reality from that which underlies the belief in universal salvation.

Universalists tend to regard Heaven as the only place of happiness and therefore a place where everybody - sooner or later - wants to go, and God as gatekeeper to this place. A loving God would - on this model - want all of his children to be happy, and would not bar the door to the place of happiness to anybody who wanted to enter.
 
In other words, universalists are unable to conceive of anybody who would not want to be in Heaven, rather than the alternatives.

In this sense, universalists regard all humans as being fundamentally alike in their nature and aspirations.

I, by contrast, see humans as having fundamental differences in their nature, and differences in what they want, from the beginning. I regard some people as having always been evil, in the sense of being opposed to Good (and Good derives its meaning from God's creation - so that to be evil is to oppose creation, and favour its destruction - to want to prevent the possibility of Good).

I presume that some souls have always been - by their nature, and for many possible - opposed to the idea of joining God's creation. This is not necessarily evil - because Heaven is an optional opt-in; a positive option.

So throughout eternity, there have been souls that preferred (for many reasons) not to join the creative and loving endeavour that is Heaven. Only some of these are the fallen pre-mortal spirits that we call demons. Others might have chosen permanent solitude, others might have chosen destruction of the Self and permanent bliss. Such choices are not necessarily irrational. 

There is also a difference in the understanding of freedom. Universalists see that any choice other than Heaven is a failure of freedom; that Heaven is the only rational choice for every person - or, that every choice other-than Heaven must be based on ignorance which, and when (sooner or later) eventually ignorance is cured - then the decision to reject Heaven would certainly be revised. And then God would (being loving) accept them into Heaven.

In sum; I believe there are some people whose nature is such that they have rational grounds for rejecting Heaven, and there are also some people whose evil opposition to creation, whose desire to see other people suffer, to inflict pain etc. is a trait they have possessed from their origins.

My understanding is that God, as our loving Father, keeps open the possibility of redemption for these people as long as possible, But a vital part of freedom is the freedom to make permanent commitments - for Good and also for evil. Indeed, this freedom seems to me vital for the existence of Heaven.

Heaven is only possible because people can make a permanent commitment to live in love, and work in participation with God's creation, with the other inhabitants - for eternity.

And the 'flipside' of this is that it must also be possible to make permanent commitments in the other direction, in the directions that involve choices other-than Heaven (some of which are actively evil, others of which are more of a passive opt-out).

So, I am not a universalist - nor do I regard universalism as evidence of God's goodness. But I can understand why, for those who accept the 'standard Christian metaphysics' of who reality is set-up, they can be led into universalism - because God's love is indeed his one essential characteristic.

However, universalism is not - long term - and in a church setting, a sustainable answer. Because when universalism is inserted within a traditional Christian framework introduces a kind of fatal flaw that tends to destroy the institutional system.

But perhaps this is itself an inevitable and necessary phase? Because if churches rely on God being seen to exclude people from Heaven who want to be there, than perhaps such a gross distortion to the essence of Christianity is so great that it invalidates any particular instantiation of the religion that depends too heavily upon it?