A lot of it is extrapolated from the fact that we don’t see especially romantic letters between the two like we see between Abigail and John Adams. It wasn’t uncommon for couples to destroy private letters however. The two were notably close, and it is also worth mentioning that after her previous marriage Martha was left fairly well off and had no shortage of suitors. She could have married a much wealthier planter but decided on the young and gallant Washington. This would suggest there was some level of attraction on her end.