It is interesting that libertarians (such as I used to be) are regarded, and often regard themselves, as being on the political Right; when in actuality they are Leftists or Liberals - indeed Leftist-Liberals of the purest type.
The transition between being a socialist and a libertarian is merely quantitative; indeed, quite smooth, logical, painless and natural - I underwent it myself and so did many others whom I have known, read about and observed.
But, in contrast, the transition between (secular) libertarian and (religious) reactionary was a wrench to undergo - profound, qualitative, and painful.
To move from libertarian to reactionary is to move from far Left to Right.
Libertarianism is merely a different means of attaining the same end as socialism. Both are utilitarian (greatest well-being of the greatest number), seeking to promote happiness and minimise suffering.
The main difference is concerning methods: socialists favour the direct system of a central command economy while libertarians favour the indirect system of market competition.
Why - if they are so similar - then is socialism so much more politically successful than libertarianism? Why is it that libertarians, when they attain power - which is rarely - always revert to socialism?
Simply because socialism is proximately more successful, more successful in the short term and selfishly - while libertarianism is an argument for long term and general goods.
Libertarians must persuade people that things can get better in the long run and further down the line only by being allowed to get worse in the short term - yet libertarians have no rationale by which to persuade people to take this nasty medicine rather than avoid it.
When the libertarian promise is based on hoping for jam tomorrow, then they will be beaten by socialists offering jam today.
Thus socialism can assemble self-interested pressure groups who are fighting for their own specific and immediate benefits (and to avoid taking the nasty medicine, which others deserve more than they) - whether material benefits, power, votes or whatever; while libertarianism is reduced to abstract, theoretical and remote benefits for society as a whole (yet without having any argument as to why anybody should care about 'society as a whole', except their own 'enlightened' self-interest - which can be served more certainly by more direct means).
The here-and-now specific good, continually subverts the abstract, distant general good - hence socialism effortlessly outcompetes libertarianism in the political arena.
Yet both are trying to do the same kind of thing. Both are on the Left, but libertarianism is further to the Left on logical grounds.
Another difference which favours socialism over libertarianism is that socialism provides a better arena for moral grandstanding - since socialism validates direct and coercive state intervention, the credit for which accrues to socialists.
Libertarians praise the market for good stuff when it happens (peace, prosperity, science, the abolition of slavery or other - real or alleged - forms of systematic oppression); but socialists praise... themselves.
...Naturally, socialism is more popular.
In sum, libertarianism is the ideal of socialism minus the graft - yet libertarianism offers no strong reason for any specific person to refuse graft. Thus, the ideal of libertarianism exists only among the powerless, and itself reinforces powerlessness.
The big difference, the qualitative difference between Left and Right, is therefore between on the one hand the Libertarian-Liberal axis which ultimately seeks worldly goods and to organize things in accordance with subjective satisfactions; and on the other hand the Religious Reactionary axis, which ultimately seeks unworldly (transcendental) Goods and to organize things accordance with reality - a different matter altogether.