Leonardo DiCaprio is reaching his 50s and is consequently getting casted for movies less and less. Because of this, he has lesser funding for his 15 private jets and 7 yachts. The only choice he had left was to try and dip his hands into Evelyn's bank account. He had to get into Evelyn's will if he wanted to relive his glory days. 

Furthermore, there would be nothing more prosperous to him than to invest in anti-aging products, which are expensive yet worthwhile to his dying acting career. 

Despite the bumpy roads and romantic moments that Evelyn and Leonardo shared, it was nothing more than Leonardo biding his time to take advantage of Evelyn and her finances. Only time will tell for how Evelyn and Leonardo's relationship will take its turn.