The doubts people may have in general about anything at all before or after this material existence are a separate matter; it is not a given that if you believe in reincarnation you don’t have doubts, or times of scepticism about the whole subject of what is often referred to as 'the continuum'.
In general once the belief of life beyond physical life is a firmly rooted proposition it becomes necessary to conjure with the possible mechanisms linking the planes of existence together.
The theory of life here as a testing or learning ground of some kind is a long held one, but as we change our morality we change our philosophy. The learning style beliefs tend to border on the semi-religious and the hang-over from this, and as we go more into physics and quantum mechanics it becomes less likely that the only reason we might come back is to learn or improve than it is simply an inevitability of some kind within the laws of physics - an unavoidable facet of the recycling process of natural energy and memory within energy.
Perhaps it's just as inevitable, for example, as driving onto a roundabout and having to drive off again at some juncture.
For some people reincarnation is not just a fanciful speculation. It is a simple deduction derived from the cyclical nature of the Universe.
If energy is invincible and we are energy then we are subject to the laws or the formation of that energy.
It becomes as obvious at certain points of speculation to believe in reincarnation as it does to breathe. The differences are ones of degree and vary with the psychology of the individual or possibly the belief system the individual subscribes to at any point of evolutionary thought or study.
Karma is a football within all this; just because there is reincarnation doesn’t mean there is karma necessarily. Some would say that the theory of karma belongs to the ‘learning/testing/remembering’ school of philosophy and is a substitute for conscience and morality in the absence of dogmatic religious code. But is it? ...
If energy doesn’t die, and we are energy, then we are fused with the occurrences and happenings which effect the ether (a subtle form of energy from which our earthly surroundings are made) and therefore we cannot escape the heritage of the ether or the photographs it contains. When we become mortally conscious or are made aware within that same ether then we are influenced by it and react to the stimulus provided from that memory:
karma as a theory and a practice is perhaps the essence of all that. As with the natural flow of the cycles, we have no choice but to be part of something larger, or more powerful, and the intensity is a matter of degree, subject to the same laws as above. In fact it could be said that those laws are karmic laws and that is what is meant by the word itself.
On a more mundane level: why do we know some people on sight, or like and dislike them without prior reason? Why do we resonate to certain locations, display extraordinary talents at an early age? Why do we yearn to be in certain places?...
All of that stuff is part of the less scientific material and might be the more convincing, if for the only reason that it appeals to the heart or emotions of us as humans.
Reincarnation as a premise feels right or it doesn’t. And when it does there is no justification or denial that makes sense except to the individual in his or her own inner journey.