Choosing the right level of income is the key for UBI to work, it has to be enough to live and survive but not so much that a recipient can enjoy luxury. Most people like to contribute to society, being is social is how humans are so dominant as a species.

Most people will contribute to the economy if they can, because it supports ambition, better lifestyle but it doesn't put pressure to worry about where today's food is going to be, people take more risks, be more entrepreneurial, explore more curiosity, explore new ideas, people spend time on acquiring more useful skills.

A mentally healthy mind is not entirely lazy. Being lazy perpetually reflects a deeper problem that is psychological to some degree such as having no hope or not being able to Imagine a happy future, or feeling helpless. Mentally healthy people want to contribute to society.

Economy as a whole will expand, which will pay in turn for UBI. First few years of UBI might be heavier on tax payers of the old system, but in long term UBI will lead to better economy. Question is not who is going to pay for it, question is can people agree to pay more out of their own pocket now for a better future for everyone? OR are we doomed as a species by exploiting our own kind?