Do these alternative thesis statements make sense and which is better?
a) Life is driven by the power of ambition.
b) Ambition is the driving force of life.
What I am trying to say is that without dreams/ambition, life is worthless, these dreams/ambitions make life worth living.
In order to sort this out for yourself, you'll need to sort out two parts of your idea first. One part is whether you mean "dreams" (i.e., hoped for future reality) or "ambition" (i.e., a strong drive for success, fame, power). These words may seem the same today since they are used imprecisely to be synonymous terms, yet these are not synonymous interchangeable terms. In fact, they are quite different from each other. They are different in meaning as the definitions from Collins Dictionary show:
a cherished hope; ambition; aspiration
[Old English drēam song;...]
1. strong desire for success, achievement, or distinction
[Old French, from Latin ambitiō: a going round (of candidates), a striving to please,...]
They are different in origin as the etymologies show: "dream" is from an Old English word meaning "song" while "ambition" is from a Latin word describing candidates going around to please powerful people in order to win positions. They are different in social connotations. To dream of future hope fulfilled has a very positive connotation while to be ambitious has a very negative social connotation that implies doing anything to get to the top position.
While these words may not seem different to you and while the social connotations may not be familiar to you, these words and their connotation may be very familiar to your reader(s), thus you have some obligation to choose which you mean and to describe what you mean precisely.
The other part you'll need to sort out is whether you want your thesis statement in the passive voice [Passive: Subject + be verb (is, was etc) + by + Object] or in the active voice [Active: Subject + Verb + Object (no connecting be verb; not connecting by preposition)]. Some automatically say always use active voice as this makes your writing stronger. The truth is that you intentionally choose between active and passive based upon what you want to emphasize, what you want to call attention to.
In this instance, if you want to emphasize and call attention to "life," then you will use passive voice construction (shown above), which puts ambition in a secondary position to the life that it drives: life driven forward is what is of greatest importance. On the other hand, if you want to emphasize and call attention to "ambition," then you will use active voice construction (shown above), which puts life in a secondary position to the power that drives it: ambition, the driver of life, is what is of greatest importance.
A clarifying example, though not in opposing active-passive construction, might be the Horatio Alger quotation: "West, young man! Go West!" Compare this to "Go, young man! Go West!" In the second, the important thing is that the young man "Go!" It is an afterthought that the young man might go West. Compare this to the emphatic endorsement of urging the young man to go to the American West, which is reinforced by the injunction to "Go!" Thus, while deciding between your two options, decide which word, "life" or "ambition," carries the emphasis of your thought.
Once you know what you are really interested in talking about--dreams versus ambitions--and once you know which you really want to put primary emphasis on (life or what drives life), then either of these two constructions will work out for your expanded purpose. In fact, these expressions and this thesis idea are long established thoughts and already have much weight in the currency of Western thought.