This is a dialogue with a correspondent.
Stephen, I cannot say I agree with the Objectivist notion you lay out that each person has a personal good.
Every individual has things which are good for them which are not good for other individuals. for example, two men with different blood groups would rationally desire to receive different bags of blood if they needed a transfusion. Human nature is not absolutely uniform, but variable.
I feel that it strays away from our Universalist commonality.
We have a commonality of nature and also diversity of nature.
If Good is in fact independent -- but not invisible to -- our reasoning, then there must be a universal good to which we all strive (or participate).
Indeed! and this good is the preservation and firm establishment of our lives. Association with God is the only prospect for this objective to be attained without compromise and to the end of time. Jesus called it "Eternal Life" and said that He came to ratify God's covenant to offer this to all who will believe and eat His body and drink His blood.
As far as I'm concerned, we must act in accordance with our nature (what we are intrinsically), and we our by our nature, rational animals.
Indeed; but my exact way of being a rational animal is different from yours because I have different aptitudes and flaws - and neutral features too. In God's house there are many chambers. There are as many ways of being holy as there are souls to be saints. Each way of holiness is a harmony of being; but each is a different harmony of being - perhaps all in the same key (I'm not sure) but as variable as melodies vary one from the other.
Aristotle's silly definition (he did rather like twee definitions; this is one - of many - reasons reasons that I am not an Aristotelian) of man as the rational animal misses out on the diversity of humanity. It also rules out without any thought the possibility that beings other than human beings might be rational animals. It also risks presuming that rationality is the same thing as consciousness, I'd say.
We can determine through reason, that there is a way we ought to be, in opposed to the way we happen to be.
Indeed, and what we ought to be is interiorly harmonious (AKA just) and directed towards wholesomeness (AKA holiness) and the pursuit of eternal life.
It would seem that as humans, we all strive for happiness, I do not believe that anybody denies this fact!
In practice yes; but what we should really be striving for is life. As the holy Torah enjoins us: "chose life". When our life is properly established, then we will be blessed and know peace and joy.
It is in our best interest to be happy,
Not if happiness is based on ignorance or disinformation. Happiness is only in our best interest when it signifies that we are "doing good". Sometimes what seems like happiness signifies precisely the opposite.
Aristotle is wrong to say (if he does) that happiness is the true end of life. Life is the true end of life. Life is "the Good" so far as living things are concerned. God is the source of life. God is the Living One.
but the question is, what makes us happy?
Many things give pleasure and effective happiness. Some are good others are bad: "heroin" and "being in love" are two interesting cases to consider. The real question is what makes us happy in a wholesome manner (as a result of obtaining interior harmony) and this reduces to the question what is wholesomeness - irrespective of whether happiness results from the attainment of it.
I feel that I must lay out that I do not think pleasure itself is a factor of happiness,
Of course it is! If I experience a lot of pleasure, I am liable to be pretty happy. Pleasure, so far as it goes, is itself a good thing - because it tends to motivate us to do things that are good for us in terms of preserving and establishing and facilitating our lives.
rather that pleasure is the outcome that happens to come with being happy.
No!!!!!! Happiness is the outcome of living well.