@NewmummyJ yes and you make some good points BUT there is also a middle ground that enables parents to help babies learn healthy sleep habits without leaving them to cry. Posts like yours unfortunately also cause huge guilt amongst parents who have felt the need to resort to sleep training. Leaving a baby to cry endlessly is of course damaging but that is not what proper sleep training actually is.
Personally I have a major issue with the attachment parenting community in general and specifically the theory that no baby should be allowed to sleep independently and that it is cruel to help a baby to learn to self-settle.
It is entirely possible to respond to your baby's needs AND gently teach them to self-settle. And as other posters have pointed out, sleep training is not just for the benefit of parents: babies and children are happier when they get decent quality sleep!
@KazMa you ask for tips. There is no need to resort to controlled crying but you do need to stop either feeding or rocking to sleep. And ideally stop cosleeping as well. Cosleeping is lovely and I did periods of it with both my younger two DC but unfortunately it does encourage a baby to simply feed on and off all night long. If you feed or rock to sleep then every time your baby stirs in the night they will be reliant on your helping them get back to sleep. Which is why your baby wakes every 45 minutes to BF back to sleep - he doesn't know how to do anything else.
Some DC do just kind of learn to self settle and resettle on their own eventually but many do not.
You can work towards self-settling very gently - all three of mine responded really well to a consistent bath and story routine with a breastfeed and then being settled in their cots. I fed to sleep for the first 2/3 months during that 'fourth trimester' period, but then gradually worked towards putting them down before they fell asleep at both nap and bedtime, and they also were put down in the cot or moses basket for very short periods when awake. By which I mean, after a nappy change I'd pop baby into the cot and put the mobile on whilst I went and washed my hands - literally 1-2 minutes, but it got them used to the space. If they were happy watching the mobile or looking at toys, I might also take an extra two minutes with them in there whilst I made the bed, or put a wash on. Never ever left to cry, or even left there for more than 5 minutes at a time. It was just about getting them used to their space so it became familiar and reassuring, and wasn't then totally alien when they were expected to sleep there! My three all loved short periods of lying in the cot awake gazing up at the mobile or looking at black and white/colourful soft books or toys etc, even as very tiny babies.
At sleep times we started using a lullaby light as a sleep association at around 12 weeks. They were never left to cry at either nap or bedtime, I stayed in the room initially, lots of reassurance etc. Different things worked better for each of my three DC - eldest responded really well to being picked up, soothed and then put down again, middle one found that unsettling and was much better just being put in the cot and shushed for a while, youngest (easy third child!) just cracked on and was quite happy to lie in the cot and listen to the lullaby light music whilst I left the room. All three were able to self-settle somewhere between 3 and 4 months and not a moment of controlled crying involved.
It will take longer for an older baby and of course they are all different and some will take longer than others and some will respond better to different approaches, as my three did. But that doesn't mean to say it's impossible. Good luck.